Frictional Electricity - Forgotten Books

FR‘

ICTIONAL ELECT RICITY

T HO MA S P T R E GL O HA N

H E A D MA ST E R

ST JA M ES’S SC IEN C E A N D A R T SC HOOL S , "E Y H A M , DE VO N PO R'

I'

F THE

L ON DON

L O N GM A N S , G R E E N , A N D C O .

1 886

P R E F A C E .

T H E present volume has been prepared with the same ideaas the ‘ T ex t-Book of Magnetism,

’to meet the syllabus of

the Science and Art Department for the E lementary Stage,

and to satisfy the requirements of the second stage of one

of the Physics C ourses of the Mundella C ode,’ as laid

down by the E ducation Department .T he same plan is followed in this work as in the

Magnetism,

’ viz.,thorough explanation

,clear diagrams

,

and numerous e xperiments to i llustrate every phase of thesubject . T he experiments are adapted for the teacher’s usei n illustrating his lectures, or for the pupil

’s private practicefor his own special benefi t.

T he diagrams Should be carefully drawn,and every

divis ion of the subj ect thoroughly mastered .

All the questions which have been set in the E le

mentary Stage since 1 86 7 are collected at the end of thebook , and numbers are attached referring to the page of thework where the explanations may be found .

A supplementary chapter has been added on ‘Suggestionsas to Apparatus,

’ and the author trusts this may be founduseful to many students .

T he,

author is indebted to other writers on electrici ty inone or two instances for short extracts and diagrams .

T . P. T .

ST . JA M E s’

s SCIE NCE A N D A R T SCHOOL S,"E Y HAM ,DE VONPOR T jzme, 1885 .

SY L L A BUS O F SC I E N C E A N D A R T

DE PA R T ME N T .

SUBJECT I X. PART II .

F z’

rsz‘ Siage, or E lementary Course.

F R I C T I O N A L E LE CTRICITY.

E lectrification by friction . T w o distinct electrical states and thei rmutual relations . Posi tive and negative e lectrification . Simultaneousand equal development of th e tw o e lectrical states . Ac tion of e lectrifiedbodies on each o ther . Conductors and non-conductors . E ffect of

connecting a conductor . E xperiments illustrating th e relation betweena primary electrical charge and th e corre lat ive induced charge . T he

Leyden j ar and similar arrangements . D i stribution of e lectricity onconductors . Action of points . Properties of hol low conductors .Frictional electrical machines . E lectric discharge.

SY L L A BUS F O R E L E ME N T A R Y SC HO O L S.

[Vew C ode, 1 88 2 .

PHYS ICS .

STAGE I I .— E L E CTR IC ITY (FR ICT IONAL ) .

At traction of l ight bodies by rubbed sealing-wax and glass.E xperimental proof that there are two forms of e lectrici ty.A ttraction and repulsion .

Go ld leaf electroscope.

Construction of electrophorus , electrical machine and L eyden j ar.E xplanation of atmospheric e lectricity.

C O N T E N T S .

INTRODUCTORY RE MAR"S .

F irst reference to E lectricitym Dr. G ilbert’s investigations— Vo l ta’s

d iscove ry— Further deve lopment— Necessi ty for experiment— Tyndall ’s opinion— Guthrie ’s Opinion pages 1—3

1 . E lectrification by F riction .

R esul t of rubbing tw o bodies toge ther— Attraction of l ight bodiesE xperz

'

ments— I nfluence of moisture on th e charging of bodiesG lass and sealing-wax representative substances— Supports for

glass and sealing-wax rods — T he E lectric Pendulum —E xperz°

~

PP 3-7

2. TWO Distinct E lectrical States and th eir Mu tual R e lation .

V i treous E lectricity— Resinous E lectricity— Posit ive and negativeE xperiments

— Theories of E lectrici ty —O ne-fluid theory— T w ofluid theory— Numerical representation of tw o-fluid theory— Numerical representation O f one-fluid theory pp . 7—12

3. Positive and N egative E lectrification .

E lectrici ties of glass and sealing-wax rods examined— Action on

charged pith balk- E xperiments pp . 12- 14

4. Simul taneou s and E qual Development of the two

E lectrical States .

O ne kind canno t be excited without th e o ther— E lectroscopes— T heE lectric Pendulum— T he P i th-ball E lectroscope— T he Gold-leafE lectroscope— T he Proof P lane— Carrier Ball — Use of e lectroscope—M e thod of charging : (a ) positively, (b) negativelyE qual quantities in rod and rubber — Faraday’s experiment

E xperiments pp. 14- 23

Contents .

5 . A ction of E lectrified Bodies on each other.

Charged rods on neutral pith ball— Charged pith balls on each o therS trips of silk ribbon , and ribbons of gutta-percha electrified— E lectrical laws— E xperiments pp . 23- 2 5

6 . Conductors and N on ' conductors .

Conductors —N on-conductors— Semi-conductors— Lis ts of each— N on

elect rics and e lectrics— Brass rod held in hand or by insulatinghandle— Insulating S too l— Human body earth-connected or insulated ~ —Method of testing conductors and non-conductors—fi C on

ductors and insulators Conductivity of wire and silk stringcontrasted Wet string Insulation Conduction E xpert

ments pp . 2 5—32

7 . E ffect of connect ing a Conductor with the E ar th .

Conductors in earth-connection and insulated— Resul t of touching conductor under inductive action of rod— M e thod of charging conductor- Induction Free E lectricity Bound E lectricity Specificinductive capacity— E xperiments pp . 32

—36

8 . E xperiments il lus trating th e relation between the PrimaryE lectrical Charge and th e Correlativel y Induced Charge .

Successive steps of inductive action— O n neutral pith ball— Go ld-leafe lectroscope— Faraday’s I ce-pail experiments— Action on conductors—T w o insulated balls— C harged conductors on each o ther— T he

E lectrophorus— Description M ethod of charging C ontinuouscharge— E xperiments pp . 36—46

9 . F rictional E lectrical Mach ines .

"inds of frictional machines— C ylinder machine— Descript ion— C harging machine— Posit ive ly— Limit of charge— Negative ly— M ethodsof showing tension— Henley’s Quadrant E lectronieter— Usem Plate

machine— E bonite or me tal plate- Me thod of charging plate machine- Winter’s mach ine— Theories of charging prime conductorE xperiments pp . 46 58

C ontents .

10 . T h e L eyden J ar and s imilar arrangements .

C ondensation of e lectricity— C ondense r explained— Three forms of

condensers— E pinus’s— M e thod of chargingu Disruptive and spon

taneous discharges— D ischarge— S low —Instantaneous -Discharg

ing rod— E xplanation of discharge— D ischarge through humanbody— S trength of charge— H ighest specific induct ive capacityFranklin ’s Pane— Charging and discharging— E xperz

'

mefzts -LeydenJar —H istory— Description -How charged —Posi tive ly -NegativelyE ffect of i nsulating j ar— Connecting inner and outer coating

with earth — D ischarge— Slowly— Instantaneously— Residual charge—Where e lectricity is stored— Jar with movable coatings

m -Chargingand discharging— Leyden Batteries— O rdinary form— Charging andd ischarging— Universal D ischarger— E ffects of discharge —Cascadearrangement— Charging and discharging— E xperz

'

ments -Unit JarUse— H arris’s— Lane ’s E lectrome ter— Condensing E lectroscopeE xpef z

'

mefl ts pp . 59-84

11. Distribution of E lectricity.

Charge on outside of conductor— M e thods of tes ting D istribution on

sphere— Cyl inder ’

-E llipso id —D isc— Cube— Density where greatest—Quanti ty, area , density, tension— Subdivision of charge— E qualspheres— Unequal spheres— Re lation be tween area and densityE lectrical tension pp . 84—89

12. Distribu tion on Hol low Conductors .

Same on sol id or hol low balls— B iot’s experiment— Hol low conductortested with proof plane— Faraday’s Butterfly Net— Cubical roomE lectroscopes protected-f—I l lustration of distribution— E xceptionsto general law— E xperiments pp, 5

13. A ction of Points .

Points attached to and presented towards charged bodies— Tension ofprime conductors reduced— E lectrical aura— Use when attached toprime conductor and rubber— Action of needle on charged rod

Act ion of point in conductor e i ther turned towards or away from acharged body— Po int proj ecting from pith ball— E lectrical whirlE lectrical incl ined plane— E xperiment; pp . 95—99

14. E lectrical Disch arges .

Three forms of spark— Luminous effects— Spark and glow dischargesS t . E lmo ’s Fire— I lluminated designs— Leyden Jar— LuminousLeyden Jar— E lectric E gg— Geissler’s Tubes —Chemical effectsVo l ta’s Pisto l — Ozone— H eating effects— Liquids ignited— Gasl i t— P latinum wire heated— Gunpowder and gun-co t ton exploded— M echanical effects— Card punctured — G lass and wood— LeydenJar pierced— M agne tic effects— Needles or strips— Action on Galvanometer— Physio logical effects— E ffec t from prime conductorLeyden Jar— Leyden Battery- Tyndall ’s remarks— Lichtenberg’sfigures

— E ffect of heat — White-hot iron ball— B right-red —Dull-red— Action of fiames— E vaporation— Induced currents— Use of flatspiral s— Secondary and tertiary currents -Duration of sparkVe locity of E lectricity I llustrations of ve locity E xperi

ment: pp . 99—1 12

15 . E lectrometers .

Use Simplest fo rms Chief e lectrome te rs Coulomb’s TorsionBalance and experiment— Pe l t ier E lectrometer— Sir Wm . T hompson’s Quadrant E lectrometa — E xperiments pp . I 12—1 I 5

16 . A tmosph eric E lectricity .

E lectricity of air— Lightning and thunder— Thunderstorm and ful

minating pane—"inds of Lightning —Thunder —C rash and pealD istance of discharge— Re turn shock -Lightning-conductors— UseArea of security— E xperiments pp . I I 5- 12 1

17 . Suggestions as to A pparatus for F rictional E lectricity.

pp . 12 1-12 5

18 . Brief History of th e developmen t of F rict iona l E lectricity.

pp . 125—12 7

Quest ions in Frictional E lectrici ty which have been set by the Scienceand Art Department from 1867 to 1885 , and references to th e pagesof th e work where the Answers may be found pp

. 128- 135

T E XT -BO O"

F R I C T I O N A L E L E C T R I C I T Y .

T HE development of this branch of physics has been theresult of the careful observation of s imple experiments byscientific minds during the last three centuries .

U ntil the reign of Queen E l izabeth,A .D . 1 60 0

,l ittle

was known of F rictional E lectricity beyond the fact that,

when amber was rubbed, i t attracted light bodies, so as tocause them to stick to i t . T his was first mentioned about60 0 B .C . by the Greek philosopher T hales

,and is considered

to be that from which electricity derived its name 5 for theGreek word for amber is etetztrmz

, in which we can see theresemblance to the word now used . After an interval ofmore than years

,D r. Gilbert

,an eminent scientist and

physician to Queen E l izabeth, gave the science a fresh impulse by his experiments with glass

,resin

,spars

,gems

,&c .

He proved that these substances, when rubbed, exhibitedsimilar effects to those which amber did. F rom these investigations the science has been gradually and successfullyunfolded by a long list of thoughtful e xperimenters, eachadding some new principle wh ich he had successfullyinvestigated

,or laying down some new law which he had ‘

conclusively established .

F rom an accidental occurrence in 1 790 A .D.,and the

reflections Of Vo lta connected therewith , E lectricity was

B

T ent-Book of Fm'

etz'

onal E

studied as the result of chemical action on dissimilar metals,

which were placed in liqu ids,and connected in circuit . T his

was the dawn of current, or as it i s known from its firstinvestigator, Voltaic E lectricity, which for some t ime madesteady progress ; but within the last few years has takenmarvellous stride s

,giving us the electric telegraph

,electric

engines,electric light

,electric railways

,&c.

What i t is destined to accomplish for mankind in thesubsequent history of the world is a problem , which the mostpenetrating intellect cannot presume to predict . I t i s without doubt one of the most mighty forms of energy

,and

,as

t ime rolls on,will no doubt be still more wonderfully de

velopedx

It should be the obj ect of everyone studying this subjectto test every phase of it by as many experiments as possible

,

that the mind may be famil iarised with the simple phenomena, which have led to marvellous discoveries . ProfessorT yndall

,in his work on E lectrici ty

,writes concerning the

art of experimenting‘ I t i s an art of e xtreme importance

,for

by its means we can,as i t were

,converse with nature

,

asking her quest ions and rece iving from her replies . Inthis way you will come into direct contact with naturaltruth— you will think and reason not on what has been saidto you in books, but on what has been said to you byN ature . T hought springing from this source has a vitalitynot derivable from mere book-knowledge .

’

Professor Guthrie,the examiner in Sound

,L ight

,and

Heat, and Magnetism and E lectricity, says in his report onthe E xamination of T raining C olleges in the annual reporto f 1 88 2 In many cases the teaching appears to have beenmainly theoret ical . Many of the answers show that thee xaminer, while understanding th e conventional description

‘

of a piece of apparatus, has probably never seen it, and cer

tainly never used it. T his won’t do and for a very smallsum many bits of apparatus may be purchased or made,and where this subject i s taught the teacher should allow the

A ttraction of L s t Bodies .

pupils to try the experiments for themselves,that they may

gain confidence, and he might inform them of the best meansof obtaining or making li ttle things for their own use .

Itherefore venture to po int out that for training collegessufficient laboratories are necessary . N o description

,how

ever conscientious, can take the place of seeing,handlin

g,and using phys ical apparatus .

’

1. E lectrification by F riction.

By F riction is meant the rubbing of one body againstanother body. When this takes place electrici ty i s producedon the bodies rubbed together

,and under certain conditions

it may be accumulated, so that i ts presence may be perceivedby its influence on F m, 1 ,

o ther neutral orelectrified bodies . T o

exhibi t this we mustresort to experiment .

A body in whichelectricity is apparenti s said to be ex eitea

’

,

ch arged, or electrified,and it w ill then attractlight bodies

,such as

bran , small bi ts of

paper, small bits of

pith , bits Of gold leaf, and other very light substances (fig .

As previously mentioned,amber was the firs t substance

tes ted, but afterwards glass , seal ing-wax, resin, gutta-percha,ebonite, vulcanite, gems, and spars were similarly examined .

Excited R od attracting l ight bod ies .

E xperiments .— T ake p iece of amber

,glass rod , ebonite rod , stick of

seal ing-wax , stick of sulphur,and shee t of coarse brown paper, and

rubbers O f Silk,fur

,and flanne l . E xcite amber with flannel , glass

with silk , ebon ite w ith fur, sealing-wax and sulphur with flannel , brownpaper w ith fur or coat-sleeve . Test each excited body by holdingthem in succession just above such light bod ies as those mentioned , and

B 2

T ext-Boole of Frictional E lectricity.

certain effects will resul t . In some cases the l ight bodies , particularlyif they have po ints about them,

will be attracted to the e lectrifiedbody, and will stick to i t . In o ther cases , as in the t iny pith-balls ,there will be attraction and repulsion , frequently repeated , causingthem to j ump to and be driven from the excited body.

N ote — I t should be stated at the outset that the moi sturei n the atmosphere has a very bad effect upon experiments in

F rictional E lectricity and,even with the greatest care

,i t

sometimes prevents an experiment being successfully performed . T o

‘

counteract thi s deposit of mo isture on thebodies to be excited they must be slightly warmed, and the

rubbers well dried. Great care i s required with glass rods,as they are more liable to thi s dewy deposit than the othersubstances mentioned. Brown paper is best e xcited

,when

it is heated until i t just begins to smoke, and then smartlyrubbed .

T wo of these substances, glass and sealing

-w ax,soon

became selected as representative substances,the former

when it was rubbed with silk, and the latter when rubbedwith flannel for i t was found that th e electricity developed

on glass and sealing-wax under these conditions exhibitedopposite effects on similarly e xcited substances when theywere brought within the influence of one another . F or example, glass rubbed with silk repelled glass rubbed withs ilk

,but attracted seal ing-wax rubbed with flannel ; while

sealing-wax rubbed with flannel repelled sealing-wax rubbedwith flannel

,but attracted glass rubbed with si lk .

In order that such experiments may be performed,the

glas s, or sealing-wax , must be supported so as to be exactlybalanced, that it may be able easily to move horizontally.

T his may be done by closing the ends of the tube in a blowpipe flame

,finding the middle of the tube by balancing

it on a knife-blade or other equally sharp edge, heating thisspot in a blow-pipe flame

,and making a slight depression

while the glass is soft,with a compass leg or other sharp

po inted instrument .

E lectric P endulum. 5

T o support it a stand may be made,consisting of a block

Of wood 3 in . square, having a hole in the centre into whicha piece of glass tubing 6 in . long and i in . in diameter i splaced . T he upper end of the tubing should be fi lled withshell-lac for about é—inq and their the hot eye of a needle

F IG . 2 .

F IG . 3.

Supports for Exci ted Rods .

pushed into the centre of the tube . T his needle-pointforms the pivot on which the supported body turns (fig.

T here i s ano ther me thod of suspending rods by means ofwire rests

,made as represented in the diagram (fig. and

fastened to a ribbon of silk for support .T he different actions of glass and sealing-wax

,when

excited by s ilk and flannel, on neutral bodies maybe plainly"

seen by their separate influences on a pith-ball suspendedby a thread of raw silk . When a glass rod rubbed with silkis brought near the pith-ball, the li ttle ball i s attracted to theglass

,touches it, and is immediate ly repelled, and will not

for a time again approach th e excited glass . If sealing-waxrubbed with flannel be now brought near the repelled pithball

,the ball will be attracted strongly towards the wax, but

in a very short time will be again repelled. An e xci tedglass rod will now attract it . By placing excited rods of

glass and wax on either s ide of the pith ~ ball i t will swingbackwards and forwards to wax and glass alternately , as i tis repelled from one and attracted towards the o ther . T his

forms the E lectric P ena’alnm.

T ext-Book of Frictional E lectricity.

E xperiment.—E xci te glass rod wi th Silk , bringit near suspended pith

bal l ; no t ice attraction , contact , then repulsion . E xcite sealing-wax withflannel , bring i t near the charged ball th e pith-ball is first attractedinto contact

,and then repel led the excited glass rod now attracts i t .

E xcite bo th glass and wax , and place them on each side of the pithbal l , sufficiently near to affect i t . L et i t firs t be attracted by th e glass

FIG . 4.

Attraction and Repu ls ion of P ith-Bal l .

i t i s then repel led to the wax , and again repelled to the glass, and soforth several t imes . Had i t been attracted to the wax first , i t wouldhave been repel led to the glass

,and from thence again to the wax , and

so on,as in the former case . Show this .Heavier bodies are also attracted by highly excited rods,

as when glass is rubbed with amalgamed s ilk, or an eboniterod i s excited with fur.

E xperiments .-Take empty egg

-shel l , which has had its contentsblown through ho les in the end

,let i t lie on table

,bring excited rod

near it as i t moves towards rod , gradually withdraw the rod, the egg

she l l wil l fo l low it .Make a cylinder of cartridge-paper 6 in . long, 3 in . diameter,

close ends , this may be drawn after excited rod similar to egg-shel l .Balance lath on rounded end of tes t tube , approach rod to end ,

not ice attraction draw i t round by the attraction of the rod .

T w o Dis tinct E lectrica l S tates . 7

At tach feather to a fibre of raw silk let i t be attracted to glass rodi t wi ll soon be repe l led ; pursue i t with the rod ; i t w il l be drivenfromi t , a nd seem to float in th e air.

' Rub sticks of ebonite , sulphur , gutta-percha , amber , &c . ,with fur

and show that they ac t similarly on th e suspended pith-ball,and

balanced lath .

H eat coarse brown paper, and rub i t briskly e ither with the coatsleeve or with a clo thes-brush , i t becomes strongly exci ted , and willat tract readily the bodies previously mentioned .

E xcite brown paper highly ; l ift i t by one corner , and place i tagainst the wall i t wil l cl ing to i t , and remain there , when the handis taken away. H o ld it over head no tice at traction of hair.Warm a drawing-board ; lay a shee t of fo re ign pap er on i t ; rub

briskly with bo t tle indiarubber if th e shee t be then lifted by a corner ,i t will cling to the -wall as th e brown paper did .

A ribbon of gutta-percha , drawn rapidly be tween the fingers , or alm of col lodion rubbed with th e hand , will show an attractive influ

ence on l ight bodies . So will a strip of narrow white ribbon drawnbe tween vulcanised indiarubber tubes placed over the fingers , and i twill also stick to the wall if placed against i t when strongly excited .

T est these

2. TwoDistinctE lectrical States and theirMutual R elation .

F rom these experiments i t will be perceived that the

e lectricity derived from glass rubbed with silk differs in itsaction on the suspended pith-ball from that developed on

sealing-wax rubbed,

with flannel, one attracting what the

o ther repels,and repell ing what the o ther attracts .

By o ther experiments i t may be seen that all electrifiedbodies exhibit influences s imilar to one or other of these

representative bodies . F or a long time it was consideredthat only one kind of electrici ty could be produced on glass ,when i t was rubbed with any rubber, and this was namedvitreous electricity from vitrum

,the L atin word for glass .

It was also thought that sealing-wax only gave one kindwhen s imilarly rubbed

,and th is was termed resinous elec

tricity,because res in formed a large portion of the substance

of the sealing-wax. R ecently, however, it has been shown

T ext-Book of F rictional E lectricity ;

that, when glass was rubbed with catskin, the same kind o f

e lectrici ty was e xcited on glass,which was thought to be

peculiar to sealing-wax,and other resinous substances also

that when sealing-wax was rubbed with gun-cotton , the

electricity considered peculiar to glass or vitreous electricitywas excited on the wax . T he names vi treous and res inousthus lost the ir significant meaning

,and new names

, positive

and negative, were substituted by F ranklin . V itreous electricity rece ived the name posit ive, or (plus) electricity ,and resinous electricity, negative or (minus) electricity.

T he excited bodies belonging to each class may be easilydetermined by testing them with the electric pendulum . T o

do this the pith-ball must be charged with ' glass rubbedwith s ilk. I t i s strongly attracted by sealing-wax rubbedwith flannel: T hese are termed unlike electricities . Allresinous substances, or other substances excited negatively

,

or charged with negative electricity,w ill also strongly

attract the pith-ball charged by glass rubbed with,

s ilk,

which is vitreous or posi tive electricity. On the otherhand, all bodies posit ively excited will act s imilarly to theglass rod rubbed with si lk

,they will repel i t .

Again, i f the pith-ball be charged negative ly by sealingwax rubbed with flannel

,the glass rod rubbed with silk

,

which is posi tively charged will strongly attract i t but anysubstance negative ly excited will repe l i t .

E xperiments .—C harge pith-bal l with sealing-wax rubbed with

flannel . E xci te glass rod with catskin ; s trong repulsion . Thisproves the e lectricities to be alike . Nex t charge pith-bal l with glassrubbed with silk , then excite sealing-wax with gun

-co tton ; noticestrong repulsion ; similar e lectrici ty. C harge pith-ball with glassrubbed with silk excite sealing-wax rubbed with flanne l ; bring i tnear charged ball strong attraction . Test ebonite rubbed with fur inthe same way ; strong attraction ; negatively charged . Rub fore ignpaper with indiarubber on a warm drawing-board , i t repel s the pithball charged with glass rubbed with silk similarly charged positivee lectrici ty . C harge bal l with sealing-wax rubbed with flanne l bringnear i t glass rod rubbed with silk ; strong attraction , positive . Teststrip of white ribbon drawn be tween vulcanised indiarubber tubes on

T ex t-Boole of F rictiona l E lectricity .

owing to two fluids exist ing in them in equal quanti ty andof enormous amount . As soon as this equilibrium was disturbed to any extent the body became charged posit ively ornegatively, according to the nature of the rubber and rubbedsurfaces .

T he process of charging he thought to be due to the

decomposition of the two fluids in rod and rubber, one of

which accumulated on the surface of the rod and the otheron th e surface of the rubber . If the rod accumulated pos it ive from the rubber

,i t gave back an equal quantity of

negative to i t,so that the amount of electrici ty remained

constant in both,but was re -arranged, one body becoming

positively,

charged and the o ther negatively , or vice versa.

Both these theories may be represented numerically, anda clearer idea of the charging of bodies conveyed to the

pupil . We will deal with the T wo F luid T heory first.Suppose rod and rubber each to contain, units of

electricity when in the neutral condition,and. that this results

FIG . 5 . F IG . 6 .

Neu tra l . R od and R ubber. E xc ited .

from both having 5 0 0 un i ts of pos it ive and 5 0 0 units of

negative (fig. L et the rod be glass and the rubber s i lk.

After a l ittle friction the neutral electricity is decomposed ineach

,and the units are re-arranged . I t i s found by experi

ment that in this case pos itive accumulates on the rod andnegative on the rubber. T his may be e xplained in th e

following way. Suppose the friction liberates one unit of

N umerica l R epresentations .

pos it ive in the rubber which passes to the rod,i t at the

same time l iberates one unit Of negative in the rod whichpasses to the rubber. E at/z still retains tlze same total units

of electricity out in an alterea’cona’ition

,the rod having

posi tive in excess and the rubber negative (fig. T he two

units of positive in the rod and negative in the rubber arenow free to affect neutral or excited bodies . If a largernumber of units be liberated by the friction

,it may be easily

perceived and numerically represented, that the rod andrubber will be in a higher state of electrical excitation by

having a larger number of positive and negative units free,

that i s remaining after the number of the units of the opposite kind is subtracted from the higher number which isproduced . In every case we must understand that the bodyis not charged entirely with posi tive or negative, but thatthere is always a certain amount of neutral e lectricity in allbodies, and that the strength of the positive or negativecharge depends upon the number of posit ive or negativeunits in excess of the equal quantities of opposite electricities,which keep a portion of the electricity of a body in the neutralcondition .

N ote— Any number of units may be assumed to reptesent the neutral condition of a body . High numbers havebeen used that i t may be easily seen how a large quantity ofneutral electricity may exis t in a body, and yet i t may behighly charged with free posi tive or free negative electricity .

I t must be remembered that whatever electrical changes takeplace in bodies

,the total number of units of electricity will

remain the same,only differently arranged . T his s tatement

will enable the student to understand many difficulties whichcould not be well understood without it .

T he'

One F luid T heory maybe numerically representedin the following way

T o account for the charging of the rod and rubber by

the‘O ne F lu id T heory,

’let us assume that each body con

tains 5 0 0 units of a peculiar fluid named electricity, and that

T ex t-B ook of Frictiona l E lectricity .

thi s i s the necessary quantity to keep each in i ts neutralcondition (fig.

If either of these receive more than this amount,i t will

be echarged with the excess posit ively,and if it lose any

,

with the loss negatively. On rubbing the rod we will assumethat it receives two units Of electrici ty from the rubber . T hecondition of each will then be represented by fig. 8

,and it

F IG . 7 . FIG . 8.

Neutra l . R od and Rubber. Excited .

will be seen that the one has more than its usual quantity,and the o ther less or

,in o ther words, one i s positively and

the other negatively charged . Any o ther units may be assumed to represent the neutral condition of the bodies

,and

higher units to express the posit ive and negative electrifica

tion besides those used here for the purpose of i llustration.

Other I deas — Some of our leading scientists considerthe electrical excitation to be due to a peculiar condition ofthe particles of the excited body, or the ether which surrounds the particles and fi lls the interstices between them .

All,however, are merely supposit ions, but afford means for

the mind to apprehend in some degree this ex traordinaryform of energy.

3. Positive and N egative E lectrification.

I t was at firs t considered that the electricity developedon glass was pos itive, and on seal ing-wax and similar substances negative ; experiment

,however, proved that the

P ositive and N egative E lectrifica l 13

character of the rubber in some cases changed the electricals tate Of the body rubbed, for negative was found to be produced on vitreous bodies, and posi t ive on res inous . T he

kind of electricity with which a body is charged may beeasily determined by testing it with the electric pendulum

.

F or this purpose the pith-ball should be charged with aknown kind of electricity— say, glass rubbed with silk, whichwe know 15 pos itive. In this condition it is strongly attractedby sealing-wax rubbed with flannel

,which is also known to

be negative . All negatively-charged bodies will act s imilarlyon the pith-ball

,charged posit ively

,as the sealing-wax does

they wil l attract i t . On the o ther hand, if the pith-ball becharged with sealing-wax rubbed with flannel

,i t will be nega

t ive,and will be strongly attracted by the pos it ive electrici ty

which is produced on a glass rod rubbed with silk . Bodiescharged with po sitive electricity also attract the negativelycharged ball . T his attraction, however, must not be reliedon as indicating definitely a different kind of electricity

,for

a neutral body woul d also attract the pith-ball charged witheither posit ive or negative but the attraction in the case of

the neutral body would not be so strong as in the othercases . I t i s better, therefore, to depend upon repuls ion thanattraction for determin ing the kind of electrici ty with whicha body is charged . We have previously seen that when thepith-ball was brought into contact with an excited rod

,i t

was immediately repelled,and would continue to be driven

from the rod, while both were e lectrified similarly . N ow,

any body charged with the same kind of electricity as theexcited rod would act similarly on the charged pith-ball i twould repel the ball . We thus conclude that when the pithball has been charged by a glass rod rubbed with s ilk, if repulsion takes place, when another charged body is broughtnear i t

,that that body is charged with pos itive electricity

or again,when the pith-ball is charged with sealing-wax

rubbed with flannel, which is negative, all electrified bodieswhich repel the ball in th is condition are negatively charged.

T ext-Book of Frictiona l E lectricityl

N egative electricity is best excited on resinous substances,

and positive on vitreous. T he best rubber for res inous bodiesis fur

,and for vitreous

,s ilk (amalgamed) .

E xperiments .-Prove al l these statements — Charge pith-bal l with

glass rod rubbed with silk for each substance tested,and not ice strong

at traction of pith-ball , when th e following excited substances are broughtnear itSealing-wax rubbed w ith flannel .E bonite rubbed with fur.

Sulphur rubbed with flanne l .Brown paper rubbed with clothes-brush .

Gutta-percha rubbed with flanne l .Ribbon of g ut ta-percha drawn be tween dry fingers .Vulcanite , or vulcanised indiarubber, rubbed with fur.

G lass rod rubbed with catskin .

Charge for each substance th e pith-bal l wi th sealing-wax rubbedwith flanne l . Not ice strong at traction from the fol lowing excited substancesG lass rod rubbed with silk .

Sealing-wax rubbed w ith gun-co tton .

Vulcanite rubbed with gun-co t ton .

E bonite rubbed with gun-cotton .

R ibbon of white silk drawn between fingers, having two pieces ofvulcanite placed over them .

Fo re ign paper rubbed with caoutchouc .Co l lodion rubbed with the hand .

Nex t,test each substance mentioned in i ts neutral condit ion with a

po si tive ly and negative ly-charged ball and no t ice that attraction alsotakes place, but not so strongly as when opposi te ly charged bodies werenear.Test , again , th e character of the e lectricity on each body in the

order already given , and satisfy yourse lf that your results are accurateby the repulsion of the similarly-charged bodies . I f in the first case thepith-ball be charged by sealing-wax rubbed with flanne l

,repulsion will

be apparent as each excited body is presented to i t , while the second l istwill repe l the ball if i t be charged by th e glass rod rubbed with silk .

4. Simultaneous and E qual Development of the two

E lectrical States .

One kind of electricity cannot be excited in a bodywithout the interchange of an equal quantity of electricity

1 5

of the opposi te sign from the one body to the other. T hismay be easily understood by reference to the numericalrepresentation of the electrical condition of a charged body,p . 1 0 . But before we examine th is portion of our subjectfully

,i t will be necessary to understand the construction

and action of E lectroscopes.

E lectroscopes .

T hese are apparatus which enable us to see by certaindefinite effects

,whether a body is excited or not and if so

,

to determine with what kind of electricity i t is charged .

E le‘

ctroscopes are usually one of three kindsa . T he E l ectric Pendulum .

a T he P i th-Ball E lectroscope .

c. T he Go ld L eaf E lectroscope .

a . T he E lectric Pendu lum.—T his has been fully de

scribed, p . 5 . T he insulated pith-ball enables us to see thata body is charged by the attractive influence which F IG . 9.

i t exerts on the baPr. Again , if the ball be chargedwith a known kind of electricity , all bodies chargeds imilarly will repel it

,while all charged with the

opposite kind will strongly attract i t . T he chargeof the pith-ball does not last long

,and so i t cannot

be rel ied on as an invariable test for any lengthof time . T he electricity i s conducted away fromthe ball by the surrounding air

,and

,if this be

damp or dusty,the discharge is very rapid . Electric

e. T he Pith-Ball E lectroscope .

"

— T his has two Pendu lum

pith-balls instead of one,and they are securely inclosed in a

well-dried glass jar or bo ttle,to prevent the discharge of the

e lectricity from the balls into the surrounding air. T hroughthe cork . but carefully insulated from it by gutta-percha,shellac, sealing wax , &c .

,passes a brass rod

, the end of

which extends into the middle of the j ar. T o this end of

the rod two pith-balls are attached by cotton , and hang atequal distances from it about two inches . T o the other

I 6 T ex t-Book of Frictional E lectricity.

end of the rod is fastened a brass ball,which receives the

electrical charge ; and thus communicates i t to the balls ,by means of the brass rod and co tton . T hese immediatelyShow that they are both charged w i th the -same kind of

FIG . 10 . electrici ty by the repulsion which takes

place . Had silk been used instead of

cotton to connect the balls they would not

have received the necessary charge,nei ther

would they have been affected by the

passage of electricity through the rod. Also,

i f the brass rod be not wel l insulated fromthe cork and bottle

,the charge will not

pass into the balls . F or thi s purpose, andas a secure means of insulation , the

P ith should be made of gutta-percha orvulcanite

,and generally the cork and neck of the vessel

coated with shellac varni sh . When the balls are charged witha certain kind of electrici ty the condition of another chargedbody may be easily detected by the movement of the balls,as the body approaches the knob at the end of the rod outsidethe vessel . Should the e lectricity of the bod y be similar tothe charge of the balls

,they gradually diverge further, as

the rod approaches but,if of oppos ite character, they

gradually collapse,returning to the ir previous position on

the withdrawal of the e xcited rod or body .

c. T he Gold L eaf E lectroscope.— T his is similar in i ts

construction to the P ith -Ball E lectroscope,as regards th e

j ar,stopper Of some insulating substance

,or cork with a

hole to insulate the brass rod,passing through it into the

vessel . T o the end of the rod with in the j ar is fastened aflat cross piece of brass about if in . long

,and on each side of

th is i s stuck a strip of gold leaf or Dutch metal about 2 in .

long and in . wide . T hese should be exactly parallel and

equal, and just long enough so that they may not touch the

s ides Of the vessel,when they are e xtended to the furthest

e xtent. T he stopper of the glass jar or bottle should fi t

exhibits by its action on the E lectroscope the kind of

électricity with whitth the body i s charged .

Sometimes instead of the Proof Plane a pith ball coatedWi th tin fo i l i s used . T his is hung by a fibre of silk

,and is

named the carrier ball .’

Use of the E lectroscope.

T he E lectroscope generally used is the Gold L eafE lectroscope, as i t exhibits most readily the presence of anexcited body ; for when a high ly charged body is somedistance away from it, the leaves show signs of divergence

,

FIG . 14. FIG . 1 5 . FIG. 16 .

E lectroscope . Divergence . Col lapse .

which increases as the body approaches (fig. but decreases again as it i s wi thdrawn (fig. T his i s in conse

quence of the charged body acting upon the neutralelectroscope . As soon as it is sufficiently near to influencethe electroscope the two electricities are decomposed, and anamount of electricity of the opposite kind is drawn towardsthe disc

,and held-there by the presence of the excited body;

while a certain portion of the same kind of electricity i srepelled into the leaves and as there are no means forinterchange with the earth , the two leaves become chargedwith s imilar electricity

,and

,l ike the s imilarly-charged bodies

previously described, they repel one another . T o determinethe kind of electrici ty with which a body i s charged, theelectroscope must first be charged with either positive or.

Cnarging t/ie E lectroscope P os itively ana’ N egatively. 19

negative electricity, and the effect noticed on bringing aposi tively or negative ly excited body near it .Method of chargingthe E lectroscOpe : (a) Positively.

Bring a negatively excited body near the electroscope,such

as sealing-wax rubbed with flannel. T his decomposes theneutral electrici ty of the electroscope

,attracting the positive

to the disc,but repell ing the negative into the leaves

,which

diverge with free negative electricity. T his free negativewill pass to the earth

,if a way be provided

,and an equal

quantity of pos itive will enter the electroscope from the earthat the same time . T hat thi s may take place

,the electroscope

must be put into conducting connection with the earth,

e ither by touching the metallic portion with the hand, or

with some suitable substance held in the hand . Whenth l S l s satisfactorily done, which is seen by the collapse of

the leaves,the hand or earth connection is removed and

then the excited rod i s withdrawn . C are must be takenthat the rod i s not withdrawn before the hand is removed ,o therwise the repelled negative will again enter the e lectroscope from the earth

,and an equal quantity of positive

leave i t for the earth , thereby again producing the equalquanti ties of positive and negative

,which restore the previous

neutral condition of the e lectroscope. In this case the

leaves remain collapsed when the hand is removed, andshow no signs of electrical charge . But if the hand bewithdrawn

,and then the rod removed, the positive electricity,

which was he ld by the presence of the rod , will spread itselfover the e lectroscope, and that amount of free posrtrve willcharge it more or less

,which will be shown by the diver

gence of the go ld leaves . If the charge be l ittle the divergence will be slight but, i f high , the leaves will spreadfurther apart .

(o) N egatively— Bring a positive ly excited body near

the disc of th e electroscope . T ouch it wh ile the leaves arerepelled with pos itive e lectricity, and the excited body is

near. As in th e previous case, positive leaves and negative

T ext-Book of Frictional E lectricity .

enters the electroscope .

.

On removing the hand, and then

the excited body, the electricity, which was attracted to the

upper portion of the electroscope , i s now free to spread

F I G . 1 8.

F I G . 19.

F I G . 1 7 .

F I G . 20 .

Neutra l . Pos itive Divergence . Earth Contact . Negat ive Charge .

over disc,rod , and leaves , and its free negative charge is

again exhibited by the divergence of the leaves .

E xperiments .—E xhibi t e lec tric pendulum , bring neutra l bodies

near,no effect on pith-ball .

E xcite rods of various kinds, bring them in succession near theneutral pith-ball ; three effects always take place : attraction

,charge ,

repulsion .

Charge pith-ball positive ly with glass rod rubbed with silk ; the

kind of e lectricity of the body to be tested can be easily perce ived as” i tis brought near the pith-ball ; if th e body be posit ively charged the

pith-ball wil l be repe l led but if negative ly charged , the ball will bes trongly attracted . Test this .Charge pith ball negative ly with sealing wax rubbed with flanne l ;

negative ly charged bodies will repel the ball, but posi tively chargedwill strongly at tract i t . Prove that this 15 so .

E xhibi t pith-ball electroscope, bring neutral bodies near the knob,and into contact wi th i t no movements of the pi th balls noticeable.

E xci te rods positive ly or negative ly bring them in succession near

th e knob not ice repulsion of balls from the free e lectricity of the samekind driven into them by the presence of the excited body. Withdrawth e rod th e balls again assume the neutral condition and cometoge ther.Bring negat ively charged body into contact with the knob ; the

E xperiments w it/z Gold L eaf E lectroscope. 2 I

e lectroscope will then be charged with negative e lectricity,and the

balls will be Immediately repe lled , having similar negative charges . I f

the body be posi tive , the e lectroscope wil l be charged positive ly,and

the balls will repe l one ano ther in this case with posi t ive e lectrici ty.

N ote — By contact the balls will have the same kind of

electricity as the electrified body has .

E xperiments .— Charge the e lectroscope zvitlzou t contact. B ring

excited body near the knob— notice repulsion of balls— touch knob or

me tallic portion of e lectroscope with hand , rod be ing st il l near,balls

again come toge the r remove hand , then remove rod the balls nowdiverge with the opposi te electrici ty to that of the exciting body : ifthat be posit ive, th e balls will be charged negatively but if negative ,the balls wil l be charged posit ive ly.

E xhibit gold leaf e lectroscope .

Bring neutral bodies of d ifferent kinds near and into contact withthe disc no movement apparent in th e leaves . E xcite bodies slightlyno tice gradual but feeble divergence as excited body approaches th edisc

,and col lapse as gradually as th e excited body is withdrawn .

E xcite bodies mo re highly : greater d ivergence is now apparent,

vio lent repulsion as th e body is near. Some t imes in this way the

leaves are broken , and / the e lectroscope spoiled . Show use of proofplane here . Take charge with small one from strongly charged rod

rub bo th sides of i t over rod , bring it into contact with the disc of the

e lectroscope : the e lectroscope i s thus charged with the same kind of

e lectricity as the excited body.

A gold leaf electroscope i s seldom charged by contact,unless the body be very feebly electrified if the body behighly charged

,the proof plane is always used .

T he action of an e xcited body on the gold leaf and pithball electroscopes should here be exhibited and contrasted,so that the del icacy and sensitiveness of the former may be

fully perce ived .

E xperiments .— Charge go ld leaf e lectro scope w itnout contact.

Fi rst,posi tively.

— Tes t i t wi th posit ive ly charged bodies , greaterrepulsion of leaves . With negative ly charged bodies col lapse , partial lyor wholly.

Second , negative ly .—G reater repulsion as negatively charged bodies

are brought near , but gradual co l lapse when the body is positivelyexci ted.

T ex t-Boole of Frictional E lectricity

In these cases,also, i t i s generally advi sable to use the

proof plane , for by means of i t a more delicate test may be

made .

T hat electricity is produced at the same time in equalquantities in the rubber and rubbed body may be plainlyshown by allowing the rubber to remain wrapped around

the rod,and placing both together on the disc of the elec

troscope . While they continue together the leaves of the

electroscope are not affected but,on

'

ivithdrawing the rod ,the leaves diverge, owing to the influence of the e lectricity

on the rubber charging the electroscope . If the rod beplaced again within the rubber, the leaves collapse . T hismay be e asily shown by rubbing sealing-wax with flannel .On removing the rod without allowing the electricity to

escape, the leaves will remain charged with the same elec

tricity as the flannel has, and on the approach of the excited

wax the gold leaves will fall together, indicating the presence

of electricity of the opposite kind, and proving the ex istence

of opposite electricities in rod and rubber .F araday arranged a sui table means of illustrating this

s imultaneous and equal development of the two electricitiesby using a rod of shellac

,and placing

over a portion of i t a flannel cap, whichhad a silk string attached to i t (fig.

On rubbing the flannel round the shellaca few t imes

,e lectricity of oppo site cha

racter was excited in both . When theflannel cap was removed by th e

,silk

string,both flannel and shellac retained

the ir respective charge s and whenpresented to the e lectric pendulum ,

ch arged pos itively,th e rod attracted the pith-ball and the

flannel repelled it. Again,when both were brought at the

same time on the disc of a neutral gold leaf electroscope, no

d ivergence took place , proving thereby that the two elec

tricities excited. in each are equal in quantity.

FIG . 2 1 .

Shel lac rod and F lanne l cap .

A ction of E lectrified Bodies on eac/z otner. 23

E xperiments .

—Rub sealing-wax with flannel ; let twax remain in

flannel, and place bo th on disc of electroscope , no movement in leavesd raw out wax , leaves diverge with pos i tive electrici ty of flannel placewax again in former position, leaves are neutralised and collapse . Rubrod again , and place both on disc as before draw out wax

,leaves are

again charged posit ively ; nex t remove flanne l from disc with vulcanisedtubes over fingers, or with tw o pieces of sealing-wax , or gutta-percha ,o r similar substances , then bring the excited wax near the e lectroscopeno tice th e leaves co llapse . Get thick round piece of she llac

,gutta

percha, sealing-wax , or vulcanite rub portion of end, abou t twoinches, with a flannel cap , having a silk string attached to i t removecap by si lk string test kind of e lectrici ty in rod and rubber by meansof the posit ively charged pith-ball of th e e lectric pendulum ,

and equalquantity in each by placing both toge ther on disc of uncharged go ldleaf electroscope no d ivergence .

5 . A ction of E lectrified Bodies on each other.

I t has been previously shown that when a pos itively ornegatively excited or charged body is brought sufficientlynear a neutral pith-ball suspended by silk the ball i s firs tattracted

,next charged with the same kind of electricity by

contact with the body, and lastly repelled . If the ball becharged

,and a heavier neutral b ody be brought near it. the

ball will be drawn to the neutral body , and be partially orwholly neutralised by the contact .When two suspended pith-balls are charged from the

same excited body,as

,for example , glass rubbed with silk,

or sealing-wax rubbed with flannel,and are brought near

one another, they repel one ano ther but,if one be charged

from a glass rod rubbed withi

s ilk,and the other from sealing

wax rubbed with flannel, they will strongly attract each other.

A striking means of i llustrating this i s to draw two pieces

of narrow white silk ribbon between vulcanite tubes placed

over the fingers . T hey will thus be charged posi tively, andwill

,when l iberated from the tubes, stand widely apart.

Strips of thin gutta-percha, drawn between the fingers, are

excited negative ly, and will repel one another from being

similarlycharged . If,however, the two positivelycharged

24 T ex t-Book of Frictiona l E lectricity.

ribbons be brought near the two negatively e xcited strips o fgutta-percha

,strong attract ion will take place, and the

ribbon and gutta-percha will In each case stick together.T hese results are always apparent under similar circum

stances— an excited body always attracts , or i s attracted bya neutral body ; a positively charged body repels a positively charged body ; a negatively charged body repels abody similarly excited ; while bodies positively and negatively charged attract one another.

T he regularity of these actions has led to the establishment of the laws, which apply to electrified bodies

,and are

termed tne electrical lazvs .

T hese laws are the following1 . N eutral bodies are attracted by, or attract electrified

bodies .

2 . L ike electricities repel .

3. U nlike electricities attract .

I n these we can trace a similarity to the magnetic law s .

1 . Magnetic substances are attracted by, or attract, bothpoles of a magnet.

2 . L ike poles repel .

3. Unlike poles attract .

E xperiments .— E xcite glass rod wi th silk Show influence on neutral

pith-ball .E xcite sealing-wax with flannel test similarly.

Charge one pith-ball positively and another negative ly ; bringfinger near them ; no t ice th e ball is attracted by the hand in bothcases they will al so be attracted by any o ther neutral body. Chargebo th balls posi tively show that they repel . Charge bo th negative lyexhibit repulsion in this” case . C harge one positive ly and ano thernegative ly ; bring them near : the balls attract one ano ther. Placetw o vulcanised indiarubber tubes over tops of tw o forefingers drawtw o s trips of narrow white si lk ribbon be tween th e tubes ; the stripswill stand wide ly apart in consequence of be ing charged with the samekind of electricity . E ach strip is strongly at tracted to a neutral bodynear it , e .g. th e wall , human body, desk , &c .

D raw tw o ends of a gutta-percha ribbon through dry fingers ;not ice strong repulsion ,

T ex t-Book of Frictional

CONDUC TOR S N O N -CONDUC TORS SEM I-CON DUCTORS .( N on-e lectrics ) . (Electrics or insu lators) .Saline solutions . D ry paper .Water . Feathers .Snow . H air.Living vege tables . Si lk .

Living an imals . G lass .Flame . Wax .

Smoke . Sulphur .S team . Resin .

Amber .She llac .Parafine .

T he two main subdivis ions may be illustrated in the

following manner. If any conductor,held in the naked

hand,be rubbed, electricity is generated

,but conveyed

along its substance through the hand, arm,and body into

the earth, as fast as it i s produced ; and no trace of elec

tricity is apparent in the conductor, even when it is testedby the most del icate electroscopes . On the other hand

,if

a non-conductor be similarly treated, the portion rubbedsoon becomes highly charged whereas the other end of thebody shows no s ign of excitation , for the electrici ty i s prevented by the nature of the substance from passing along it .F rom these two oppos ite effects being apparent in all bodies

,

they were at first divided into non-electrics and electrics,the

former corresponding to the conductors, and the latter tothe non-conductors .

N on-electrics were so named because it was thought,at

that t ime,that they could not be electrified . Subsequent

investigation, however, has shown that, if such bodies be“

separated from the earth by means of a non-conductor or

insulator, or if they be, as i s usually termed, insulated, whatwere known as non-electri cs may be excited .

A brass or other metall ic rod, held in the bare hand

,

canno t be e lectrified, if violently rubbed for a long time ;but if it be fastened to an ebonite, gu tta-percha, glass , orother

non-Conducting handle (fig. or if one end be wrapped

2 7

around carefully with Sheets of vulcanised caoutchouc,or

layers of s ilk, the metallic substance may soon be e xcited byrubbing . A conductor, however, although it beinsulated, will not become so highly charged as a

non-conductor when similarly rubbed, for in theformer case the electricity, as soon as it i s generated,spreads over the whole surface of the conductor ;whereas in the latter case it i s confined to the smallportion of the non-c onducting substance which hasbeen excited . T he charging of a non-electric or

conductor may be clearly seen by striking the disc

or ball of an electroscope with fur or flannel . E lec

tricity is thus generated and conveyed through the

rod, which is insulated in passing through the cork to Bras s

the leaves . T hese gradually diverge in consequence”81g?

of the charge communicated to them,and the kind handle

of electrici ty may be tested by methods described in p. 2 1 .

T he human body is also a good conductor and,while

in contact with the earth, allows the electricity a free passage

,and shows no trace of charge

,however rapidly it may

be generated ; but if the personbe placed upon an insulating stool ,which is a mahogany board on four

glass Supports (fig . 2 and then be

struck with flannel,fur

,or a fo x ’s

brush,he becomes so charged that

on presenting the hand to the discof th e electroscope th e leaves arevio lently repelled . On bringingthe knuckle near the pith-ball of the electric pendulum

the ball is first attracted,then charged, and afterwards

repelled by the similar electricity of the insulated body .

Or,agaIn, if another person, standing in earth contact,

bring his knuckle near any part of the charged insulatedperson ’s body, sparks may be drawn from him or her. Asheet Of gutta-perch a, caoutchouc, ebonite, or vulcanite, or

F IG . 2 2 .

F IG . 23.

Insu lating Stoo l .

T ex t Boole of Frictional E lectricity .

a disc of any non-conducting substance placed on the '

ground for a boy or girl to stand upon,will serve the same

purpose as an insulating stool ; or a dry , warm drawingboard placed on four dry glass tumblers will also do .

T he best means of exhibiting the difference between thepassage of electricity through a conductor and an insulator

,

is to charge an e lectroscope, and touch the disc with substances selected from the list of conductors mentioned on

pp. 2 5 , 2 6 . T hese cause the leaves to co llapse instantly,

showing that the electrici ty,which previously charged the

electroscope,had been conducted through the conducto r

and the body into the earth . T he electro scope must becharged separately for each of these experiments . If theelectroscope be again charged for the testing of the non-con

ductors , and substances be taken from the l is t and presentedto the disc, as in the previous case, no movement whateveri s noticeable in the leaves they still remain charged

,prov

ing that no electricity passes away through the body whichtouches the electroscope.

All substances may be easily classified by submittingthem to this test . I f it cause the leaves of the electroscopeto collapse instantly, we may certainly consider i t to be a

good conductor ; but if the leaves remain divergent, and

slzozv no signs of collapse, the substance i s as certainlyproved to be an insulator.

Should the charge be observed to pass away gradually,

not rapidly, as through a conductor, or he apparentlychecked

,as by a non-conductor

,th e substance should

then be included amongst those bodies which are knownas semi-conductors .

T he difference in the passage of electricity for a distance

of several feet or yards through a conductor or non-condue

tor may be s trikingly i llustrated by usrng a long thin copper

w ire,and a string of s ilk of the same length . E ach should

be attached in turn to an electroscope by one end, and wound

around a glass or ebonite rod by the other. On exciting

I nsu la tion .

therod with the rubber, while rod and rubber are In con

tact no movement is apparent in the leaves of the electroscope in either case but, as soon as they are separated theleaves diverge instantly, and widely, when the wire is usedbut they are unaffected when silk is the connecting mediumthus proving that the wire conducts the electricity, but thesilk checks or resists its passage. C are should be takenthat pure silk i s used

,otherwise the leaves will show a gradual

divergence,owing to i ts becoming a semi-conductor to a

certain extent by its mixture with cotton .

T he conductivi ty of string, when it is we t or dry, maybe s imilarly shown ; and from this the effect of dampnessand -mo isture perceived . T he influence of a conductor anda non-conductor may also be contrasted by using the

electric pendulum to exhibit the latter, and a pith-ball suspended by co tton from a wire, bent similarly to the stand of

the pendulum,but in earth contact for the former. T he

pith-ball of the electric pendulum is first attracted to anexcited body, charged, and then repelled whereas the ballin earth contact is attracted to the excited body

,discharges

the whole of the electricity if the body be a conductor, or aportion of i t if it be a non-conductor, after which the pithball is l iberated in its neutral condition , as at first .

I nsulation — T o enable a conductor to become chargedwith electrici ty, i t must be insulated, that is, i t must beseparated from the earth by some non-conducting substance.

T his is generally done by supporting conductors on glassrods

,and in order to render the insulation complete

,the

glass rods should be coated with shellac varnish to preventmo isture be ing deposi ted upon them from the atmosphere .

On account of the hygroscopic nature of glass, under certainconditions mo i sture i s deposited in a dewy fi lm very rapidlyupon it. T his soon destroys the insulation , by changingthe insulator into a conductor . Varnishing glass rods is aremedy for this . With every precaution, however, a con

ductor does not hold i ts charge for any cons iderable t ime,

30 T ext-Book of Frictional E lectricity .

for the mo i sture and the tiny dust particles in the atmosphere assist in gradually reducing the charge.

Some substances, as ebonite,.

gutta-percha,sealing-wax

,

&c ., insulate better than glass, as they are not so hygroscopic ,

and require no shellac varnish to be used on them .

C onduction .-T he first e xperiments on conduction and

insulation were performed by Stephen Gray. At first henoticed that a cork stuck in a glass rod attracted light

bodies.

when the rod was excited . H e thenstuck rods of deal about six inches long intothe cork

,and these also became electrified

on exciting the glass rod. F rom these he

passed to packthread,and found that light

bodies were attracted at the end of longlengths of thread. He increased the lengthsof the thread

,and still the attraction was

not iceable . In one experiment he usedupwards of 70 0 feet, and suspended it withthread, but no electrici ty passed through it .

When silk loops were used to support itBrass ba l l attachedto exc ited rod by Instead of thread

,the electrrcrty passed

w ‘re '

readily to . the end of it. On substituting athinner wire loop for one of the silk the electricity failed to

reach the end of the thread. He then came to the conclusionthat the silk prevented the passage of the electricity to the

earth, and so confined i t to the thread but the wire loop ,when it was in contact with the thread, permitted the electricityto pass to the earth by that means . H e thus discoveredthat the s ilk was an insulator

,but the wire a conductor.

C ontinuing his e xperiments, he also examined o ther

substances, and arranged them under two divi sions, insula~

tors and conductors.

E xperiments .—Rub brass rod held in the hand with fur touch disc

of the e lectroscope no movement of leaves conductor,non-electric .

Rub ebonite rod with fur ; bring it near the electroscope ; leavesd iverge non-conductor or e lectric .

E xperiments w itn Conductors and N on-Conductors . 3I

C harge the e lectroscope for each experiment w ith e i ther kind ofe lectricity ; touch the disc with as many substances as possible fromthe l ist of conductors ; the leaves co l lapse Immediately the disc istouched e lectrici ty conducted away to the earth bodies conducto rs .Charge e lectro scope again for experiments with non-conductors ;

touch the disc with as many as possible of those which are given inthe l ist no co llapse of leaves e lectricity not conducted away from the

e lectroscope substances non-conductors .C harge e lectroscope again , and show gradual d ischarge , when the

disc is touched with th e semi-conductors .Rub brass rod with fur no movement in leaves when it is brought

to the e lectroscope . Fix insulating handle to brass rod , such asebonite , gutta-percha, or glass , orwrap one end in shee t-caoutchouc , orin layers of silk brass rod may now be excited by rubbing, or s trikingrapidly with fur. Rub with fur touch e lectroscope leaves d iverge .

Ho ld e lectroscope in the hand strike the disc a few t imes withflannel , fur, silk , &c . , no tice divergence of the leaves , as the e lectroscope becomes charged .

L et a lad stand on the ground , and hold his hand j ust above the

disc of th e e lectroscope , then strike him with a fox ’s brush or with fur,no divergence takes place . Place him on the insulat ing stool , andstrike him similarly leaves now dive rge free ly . Try i t also with ladstanding on a shee t of girtta-percha or vulcanised Caoutchouc .Bring knuckle near e lectric pendulum no tice effect . L et another

lad present his knuckle tohim , and take a spark .

Fasten long thin copper wire to e lectroscope ; wrap o ther endaround ebonite rod rub rod , and not ice condit ion of leaves , as rubberremains in contact with rod separate rubber from rod strong divergence .

At tach silk string sim i larly, after excitat ion , and separation of

rubber from rod no divergence of the leaves whatever.Test dry string and w et string similarly

,and no t ice effects .

Show the need of insulat ion for a conduc tor .P lace a conducto r on a brass support , no charge can be imparted to

i t . Test again , when supported on a clean glass rod the conducto r ischarged , except under very favourable conditions , with difficulty.

P lace conductor on glass rod coated with she llac varnish , chargede asily. No t ice this . E xplain and understand the cause of these different effects .Push cork into the end of a glass tube ; excite tube ; e lectricity

also charges the cork show i t by repel ling similarly charged pith-ball ,and by its action on th e leaves of th e e lectroscope .

T ex t-Boob of Frictiona l E lectricity.

S tick rod of deal about 6 in . long into the co rk ; rub tube againShow repuls ion of pith-ball , and divergence of the leaves , as before ,when wood is brought near them . Suspend small brass ball at the endof a wire , and again at th e end of a si lk st ring

,and show the ir effec t

on an electroscope . Attach long length O f thin copper wire to electroscope and rod . E xcite rod , leaves diverge , touch wire in any placewith conductor ; not ice co l lapse of leaves touch with insulator, leave sunaffected .

7 . E ffect of connecting a Conductor with the E arth .

In the last division of the subj ect i t has been shownthat

,in order that a conducting substance may be electrified,

i t must not be in contact, or in conducting connection withthe earth

,but must be insulated . T hat a conductor also

may retain tbe cbarge which it receives from the action of anelectrified body, i t must be kept from cart/i connection by

insulation. T he accompanying diagrams will make thi s clear.L et the positively excited rod, A , be brought in succession

near the conductors B and 0 , one of which (fig. 2 5 ) i s sup

F IG . 2 5 . FIG . 26.

Conductor on brass support . Conductor on glass connectedW I th earth by Wire .

ported on a rod of brass, and the other (fig. 2 6) on a rod of

varn ish ed glass, but connected with the earth by means of

a wire.Both of the conductors are thus in earth contact .

N ow ,as soon as the e lectrified body is brought near enough

to act upon the conductor, the first effect produced is to

d ecompose a certain amount of neutral electricity in the

conductor ; negat ive, the opposite kind of electricity, is

T ent-Boole o"F rictional E lectricity .

connection with the earth or any conductor larger than

i tself.T he posit ive is l iberated by the presence or the positively

e xcited rod, and is thus rendered free to pass to the earthor other large conductor while the negative i s held captiveby th e attractive influence of th e rod, and is unable to movefrom the part of the conductor, where it is produced, until

the rod be w i thdrawn or discharged . When in this condition

it is termed bound.

I t may here be stated that the electrical action which takesplace between an excited body and a conductor

,when they are

not in contactwith each other,is named I nduction

,and that the

decomposed electricity of a conductor,when under the inductive influence of an electrified body, i s either free or bound.

F ree E lectrician— E lectricity is termed free, when it is sofar liberated from the neutral condition in which i t previouslye x isted, that it is enabled to exhibit its attractive andrepell ing influences on outside bodies , or to pass to theearth or other conductor

,i f a suitable conducting means be

provided for i ts passage. T he charge excited on a rod byfriction is free electricity so i s that which causes the leaves

of the electroscope to diverge, and the electricity whichpasses to the earth from a conductor through the inductiveaction of an electrified body, so also is that which is apparentat the furth er ex tremity of an insulated conductor whenunder induction

,and the charge of the prime conductor of

the electrical machine .

B ound E lectricity— E lectricity i s considered bound when

,

after it i s separated from its neutral condition,it i s held cap

tive or paralysed by the action of some electrified body neari t,and is thereby rendered powerles s to exert its influence

on outs ide bodies . Under certain conditions, on the with

drawal of the electrified body, the l iberated e lectricitycharges the conductor , and is immediately transformed frombound to free for then it is capable of affecting other chargedor neutral bodies .

Specific I nductive C apacity . 35

T he electricities of both rod and rubber are bound, after

they are rubbed together, until separation takes place . T he

electricity on the disc of the electroscope i s bound while i ti s under induction

,and will not pass away although the disc

be brought into earth contact. T hat also which is produced

on the end of the conductor near the inducing body is alsobound and so are the electrici ties on each side of a con

denser or L eyden j ar. Bound electricity is always of the

opposite character to that which retains i t .

Specific I nductive C apacity — I t was established by Professor F araday, that for an excited body to act upon a con

ductor by induction,some substance must exist between the

two through which the electricity may be imparted . In ourexamples on induction

,air is understood to be present.

Some substances , as, for example , varnished glass, sealingwax

,ebonite

,&c . ,

allow this action to take place at thesame dis tances much more free ly than air. T hose substances which enable electrici ty to act most s trongly throughthem are said to have the big/test specific inductive capacity.

Dry air i s superior to mo i st air in thi s respect .A conductor may be charged either positively ornegatively

s imilar to the method of charging the electroscope, pp . 1 9, 2 0 .

E xperiments .— P lace conducto r on brass rod test i t with e lectro

scope ;'

no action on leaves . Bring posi t ive ly exci ted rod near conducto r ; withdraw it test conductor again with e lectroscope : no charge .

Support conductor on insulat ing rod of varnished glass,ebonite, or

o ther non-conducting substance attach thin copper wire to the end of

the conducto r and the disc of the electro scope . Bring excited rod nearthe conductor ; no t ice the divergence of th e leaves of the e lectroscopeas the rod approaches . Withdraw rod no tice gradual co llapse of the

leaves as the distance increases . Refer to similar action taking place inal l bodies under induction , al though in most cases invisible . Repeatthe last experiment , and while th e leaves are divergent remove th e

wire with some insulating substance , such as vulcanite tubes overfingers ; test th e character of th e charge ; show it to be posi t ive or

negative according as the leaves become mo re dive rgent by a glass rod

rubbed with silk , or stick of sealing-wax exci ted with flannel .L et pith-ball rest against the end of the conductor away from the

D 2

T ex t-Boob of F rictiona l E lectricity .

portion nex t the rod ; bring excited rod near conducto r ; notice the

repulsion of the pith-ball,being charged with the free e lectrici ty of the

conductor . Withdraw rod , e lectricit ies recombine conductor neutralised , ball also neutralised , and is at tracted towards conductor, afterwhich i t hangs vertically as at first . Repeat a portion of this experiment , and while the pith-bal l is repel led by the free e lectricity

,remove

i t from the influence of the conductor , and test i ts character , as in theprevious example . Bring glass rod rubbed with silk to insulated conductor ; touch the conductor in any part with the finger while i t isunder induction , remove th e hand , and then withdraw the exciting rodth e conductor will remain charged with the opposite e lectrici ty fromthat of th e exciting body. Test this by showing the repulsion of th e

pith-bal l of the e lectric pendulum,charged negative ly take a portion

of i t wi th a proof plane , and test with an e lectroscope charged negatively leaves d iverge further— same e lectricity.

8 . E xperiments il lustrating the relation between the

Primary E lectrical Charge and the CorrelativelyI nduced Charge.

As explained in the last divis ion of the subject,an ex

cited or charged body acts inductively on a neutral body,

and the successive steps of that action are

1 . T he decomposition of the neutral fluids .

2 . T he rearrangement of the two electricities the

opposite kind be ing attracted towards, and the same kindrepelled from the electrified body.

Should the charged body be withdrawn,the two e lec

tricities recombine , and the conductor returns again to i tsneutral condition . T his may be plainly shown by the action

of a charged body On the gold-leaf electroscope . As thebody approaches the e lectroscope the leaves graduallydiverge

,but collapse again as i t i s withdrawn . A similar

change in the condition of the electricity of the body must

be imagined under all cases of induction .

We will examine first the action of a posi tively excited

rod on the pith-ball of the electric pendulum (fig . As

th e rod approaches the pith-ball, near enough to affect it, the

first influence i s to decompose the neutral electricities of the

I llus trations of I nductors . 37

ball,and the next i s to rearrange them negative electrici ty

i s attracted to the part of the ball nearest the rod,and

positive i s repelled to the p art away from it . N ow,from

the action of the law of i nverse squares,the attraction is

greater between the posit ive rod a nd

the negative part of the ball,than

the repulsion between the rod andthe posit ive part

,and therefore the

ball is attracted towards and touchesthe rod . I t then gives a portion of i tsnegative to

,and rece ives an equal

amount of positive from the rod, andthus becomes charged with a certainquantity of free posrtrve . T he ball i snow repelled from the rod

,and this

repulsion continues until the free positive is discharged, eitherby the moisture of the atmosphere

,the dust particles in the

air, or by touching some body which permits its discharge.T his i s what i s known as charging by contact.An electroscope is s imilarly acted upon by an excited

body,as may be understood from the accompanying dia

gram (fig.

If the rod be brought into contact with the disc or ball ,equal quanti ties of negative pass from the electroscope to

FIG . 30 . F IG . 31 .

F I G . 29.

Action on Neu tra l P ith-Ba l l .

Action on Go ld Leaf Electroscope . Inductive act I on from Proof P lane .

the rod and posit ive from the rod to the electroscope. T he

electroscope thus becomes charged with a greater quantity

T ex t-Boob of Frictiona l E lectricity.

of free positive, and the leaves diverge further in conse

quence . With a highly charged rod this is a dangerous ex

periment to attempt,as i t i s l ikely to break the leaves

,

particularly if the electroscope be a sensit ive one . T o avo idthis

,the proof plane is used

,and by means of i t small

charges may be removed from the surface of the chargedbody to be tested by the electroscope (fig.

When this is done,the influence on the electroscope is

s imilar to that exerted by the rod , but the leaves are muchl ess vio lently disturbed.

F araday’

s I ce-pail E xperiments.

T he c haracter and quantity of the induced electricitymay be clearly seen and easily detected by what is known asF araday’s Ice-pail E xperiment. T he ordinary ice-pail i s

F I G 32 made Of pewter,and is about seven inches

in diameter and ten inches deep (fig.

T his must be insulated,and connected on

the outs ide by means ofa copper wire with agold-leaf electroscope . On bringing a posItively charged brass ball, held by a longsilk string

,into the interior of the ice-pail,

negative electricity i s induced on the innerO ne Insulated I ce-pa i l . and positive on the outer surface of the

vessel . T he free positive of the ex terior of the pail causesthe leaves of the electroscope to diverge gradually as theball i s lowered into it, until it has sunk about three inches intothe vessel

,after which no further divergence takes place .

If now the ball be. lowered, so as to touch the bottomof the pail

,and then be withdrawn , the ball will have become

neutral ised by interchanging with the negative of the pail,but the leaves of the

.

electroscope will not diverge furtherthus proving that the charge, produced on the outside of thepail by induction from the charged b all, was equal to thatwhich it re ceived by contact with it .

F araday varied the experiment by using four ice-pails,

Faraday’

s I ce-pail E xperiments . 39

placed in one another (fig. but separated from eacho ther by layers of shellac, thus securing the necessary insulation . T he outside of the outer pail was again insulated

,

and connected by a thin copper wire with a gold-leaf electroscope. When the charged ball was brought into the innerpail of the serI es, the leaves of the electroscope diverged inthe same manner as they would have done if only one pail

were used. T his arose from the inductive action of the

inner p ail on the one adjacent to i t, that one on the next,

8 m,until the free posit ive of the outer vessel through thewire connection charged the electroscope, and caused theleaves to diverge. If now the ball be withdrawn

,the

leaves collapse, showing that the decomposed electricities inthe pails had again recombined, and the series returned

once more to the neutral condition . If,however

,before

w ithdrawal, the ball be allowed to touch the inner pail,

i t is charged with the same kind of electricity as the charge

of the ball, and, as in the previous case, the ball i s neutral ised,but the leaves show' no further divergence .

If now the inner posit ively charged pail be removed bysome insulating substance, the remaining pails are neutral ised ,and the leaves immediately fall to F I G-33.

gether,but diverge again on i ts being

replaced. If the series be connectedin order from the outer pail, as 1 and2,2 and 3, 3 and 4, or 1

,2, 3, 4, no

alteration in the leaves of the electroscope will be apparent .Again if

,while the pails are under

the inductive action of the charged Four Insu lated I ce pai ls .

ball,the Inner pail

, 4, be connected with the earth, th e free

leaves. T he negative of the inner pail i s unable to disturb

the neutral condition of the other pails, because it is heldbound by the presence of the positively charged ball.

40 T ex t-Boob of Frictional E lectricity.

Should 3 be connected with the earth , 2 and 1 immediatelybecome neutralised ; and, lastly, i f 2 be placed in earthconnection, 1 i s restored to the neutral condition .

A ction on Conductors .

A cylindrical conductor may be used to exhibit the in

fluence of an excited body upon it by attaching a small electric pendulum to eachend" of i t . Suppose apositively charged rod

be brought near the conductor

,the pith-balls on

the ends immediate lyrise— the one nearest therod charged with negative electricity

,and the

E ffect of the presence of a Pos itive ly one awayfrom it ChargedE x C ited R O d with

.

positive (fig.

If the rod be withdrawn , the pith balls instantly fall, and theconductor is again neutral ised . Should a negatively charged

rod be used, the pith balls also ri se, but in this case chargedwith opposite electricitiesto the former example (fig.

On the withdrawalof the rod the two electricities again recombine .

T he kind o f electrici tycharging the pith-balls ineach case is shown by therepulsion

,which is notice

Effect of the presence of a Negative ly able When they are t€St.

€dE x C l ted R od by a glass rod rubbed wrth

silk and a sealing-wax rod rubbed with flannel .When two equal brass or tin-foi l-covered wooden spheres

are i n contact and acted upon inductively by an excited body

one ball becomes charged pos itively and the other negatively

F I G . 34.

F IG . 35 .

T ex t-Boob of Frictional E lectricity.

one another (figs . 38, the decomposed electricity of thefirst influences the next, and this the next, &c .

,s imilarly to

the action of a magnet on neighbouring pieces of soft iron.

(See Magnetism,p .

T he E lectrophorus .

A most important piece of apparatus for exhibiting theinduced charge obtained from the action of the primarycharge is the electrophorus . T his cons ists of two circularplates, one of which i s about 2 inches less in diameter thanthe other. T he larger forms the base

, or portion of the

F I G . 40 .

FIG. 41 .

P L A T

apparatus to be excited, and must be of some insulatingsubstance— generally a cake of resin or ebonite— containedin a tin dish which is named the ‘ sole . ’ T his forms tlze

FIG .

~

42 . FIG. 43.

generatingplate. T he smaller i s the conductor, and must beof metal . It i s generally made of brass or tin. T his isknown as tne collecting plate. T O this plate an insulatinghandle is attached

,and the apparatus i s complete .

C /zarging E lectropnorus . 43

Method of Charging — T o charge the electrophorus thecake must first be excited. T his is usually done by warming its surface and then striking i t smartly two or threetimes, or rubbing itbriskly with fur or

flannel,which causes

i t to be electrifiednegatively. T he con

ductor,held by the

insulating handle, i snow lowered graduallyupon it . As this ap

proaches, the electri

city of the base actsinductively on the conductor

,decomposes the neutral

electricity of the metal plate,and attracts posit ive electricity

to the lower face, but repels negative to the upper . Shouldthe conductor be withdrawn again without being touchedwith the hand

, orantherwise brought into earth contact,

the two electricities will recombine,and the neutral condition

be again produced .

But if,while the conductor rests upon the excited cake

and is under the inductive action of i t, the upper surface of

the conductor be touched by the finger,the free negative

passes to the earth,and an equal quantity of positive enters

the disc from the earth . T he hand should then be removed,so as to break the earth connection, and the conductor be

l ifted by the insulating handle . Itwill then be found to behighly charged with free posit ive electricity, and if the

knuckle be brought to the edge of the disc a spark will be

seen,and a feeble crack heard

,which is the result of positive

passrng from the disc to the knuckle, and negative at thesame time passing from the knuckle to th e disc. If a pithball

, or a number of pith-balls, be laid on the conductorbefore it i s charged, on charging it as described and liftingit from the cake , the pith-balls are immediately and violently

F IG . 44.

Pos it ive ly charged Disc.

T ex t-Boob of F rictiona l E lectricity.

repelled, being charged with the same kind of electricity asthe conductor. T he kind of e lectricity charging the disc maybe tested by bringing it near an electric pendulum or goldleaf electroscope charged positive ly . T he free positive of theconductor may also by means of several charges be employedto charge a L eyden jar. T hese success ive charges may beobtained from the same excited cake

,for as the charging is

by induction and not by contact, l ittle electricity is lost bythe cake when the disc is charged . Ot his po int

,Professor

Guthrie writes Scarcely any actual interchange betweenthe of the brass and the of the cake takes place, because the two surfaces only touch one another in a fewpo ints

,and the resinous substance

,being an exceedingly bad

conductor, does not permit the recharging of the dischargedpoints . ’

T o avo id the necessity of touching the collecting plate ofthe electrophorus every time it is required to charge it, abrass pin

, or a series of pins, may be brought from the solethrough the cake to i ts surface, so as to be in contact withthe upper disc when it is placed upon the excited cake . Inthis way the free negative escapes from the collecting plateto the earth through the pin or pins

,and an equal quantity

of free positive passes by the same means from the earth to

i t . Sometimes a strip of tin-fo i l i s brought from the soleover the cake to mee t the edge of the upper disc, and thisoffers a similar passage for interchange with the earth as thepin arrangement does . In this way a rapid succession of

charges may be communicated to,and sparks obtained from

the collecting plate of an electrophorus, and a L eyden jarbe charged most quickly from it .

E xperiments .— E xcite glass rod with s ilk bring i t to the d isc or

knob of th e e lectroscope no t ice divergence of the leaves . Wi thdrawit the leaves col lapse .

E xcite sealing-wax with flanne l tes t similarly sim ilar results .Tes t e lectric pendulum with exci ted glass and exci ted wax no tice

movements o f pith-ball in each case .

E xperiments on I nduction . 45

T est electroscope with highly-exc ited rod the leaves d iverge whenthe rod is some distance away.

D raw a proof-plane over excited rod bring it near to, oron disc orball of elec troscope similar act ion apparent .Obtain me tallic vesse l 7 inche s in diame ter and 10 inches deep .

Insulate i t . At tach wire to outside of the vesse l and to the disc of

e lectroscope . Not ice divergence as posi tive ly charged ball is lowered ;w ithdraw bal l leaves co llapse . Lower bal l again , and let i t touch thebo t tom of the vesse l no increase in d ivergence apparent after the bal lhas sunk be low 3 inches .

_,I f possible

,tes t four similar vessel s, but of slightly different sizes, and

insulated from one ano ther and th e earth . Charge th e inner one positively : the leaves of the e lectroscope diverge with posit ive e lectricity.

Remove this charged inner vesse l by means of si lk handle s, which shouldbe fastened to i t with sealing-wax leaves col lapse o ther vessel s neutralised . Connect in succession , while under inductive influence of chargedbody

, 4, 3, 2,1 with the earth the leaves co llapse— due to free

e lec trici ty from each interchanging with earth and neutralising outerve sse ls .Fix a small e lectric pendulum at each end of a conductor ; bring

excited rod near th e pith balls are repe lled . Test kind of e lectricityof each ball by using posit ively and negative ly charged rods no t ice re

pulsion similar e lectricity.

B ring two equal spherical balls into contact let an excited rod be

brought near them ,so as to decompose the e lectrici ty. Remove th e

further ball,then withdraw th e rod th e tw o balls will be charged with

opposite e lectricities ; that further away has similar e lectricity to the

excited rod,and that near the opposite kind . Tes t this by means of

proof-plane and e lectroscope . Charge balls again in this way,then

bring them very near to one another a feeble spark will be visible , andthe ball s become neutralised .

E xcite cake of e lectrophorus by rubbing with fur or flanne l , or

striking with a fox’s brush bringconducto r on excited cake , and withdraw it again without having brought i t into earth contact ; test withe lectroscope no t race of e lect ricity .

B ring conductor again on excited cake now touch i t with finge r,remove finger

,and lift conductor by insulating handle present knuckle

to conducto r spark passes .Charge conductor again similarly, bring disc towards the e lectro

scope no tice divergence of the leaves when it is some distance away.

C harge e lectroscope with positive e lectricity ; the leaves divergefurther as the disc is brought near it . C harge ano ther e lectroscope

T ext-Boob of Frictiona l E lectricity.

negatively the leaves co llapse gradually as the disc approaches . T he

proof-plane Should be used if the charge is very high .

Fasten thin copper wire to the disc of th e e lectrophoru s , and attachth e o ther end to th e e lectroscope ; no tice movement of leaves as thedisc is brought on th e excited cake ; collapse as disc is touched , andstrong divergence as disc is l ifted from cake .

Bend tin-foi l from so le over edge of cake to mee t disc excite cake,and obtain a succession of charges from excited cake charge L eydenj ar by means of a series of sparks from charged disc .

9. F rictional E lectrical Machines.

T here are two kinds of frictional electrical machinesthe cylinder mach ine and the plate machine . T he essentialparts of each are the rubber, the cylinder or plate on whichthe electricity l S excrted , and the prime conductor on whichi t is accumulated . In these respects they resemble thee lectrophorus, which is the first form of electrical machine .

T he Cylinder Machine was first developed by,and its

present form is due to,Gordon wh ile the ordinary plate ma

chine owes its origin some say to Planta,others to Van Marum .

A later varie ty of plate machine was developed by Winter.T he Cylinder Machine — T he rubber Of the machine

consists of a long pad of horsehair covered with soft leatherabout three-quarters of the length of the cylinder . Beforeuse it is smeared with bisulphide of tin

,or an amalgam con

s isting of one part of zinc, one part of tin , and two parts ofmercury

,laid on with lard, to develop the electricity more

freely on th e surface of the cylinder or plate . T he rubbermay be insulated by being fi xed on a glass support coatedwith shellac varnish, or i t may be fastened to a wooden stand ,and thus be in earth connection . If the rubber be insulated

,

in order that the prime conductor may be charged, a wire or

chain must pass from it to some conducting connection with

the earth , as e.g. the ground, gas-fittings, &c .

,or the chain

may be held in the hand of a person standing on the ground .

A flap of silk extends from th e rubber almost to the

prime conductor to prevent the electrici ty escaping into the

T /ze Cylinder M ac/sine.

atmosphere in its passage from one to the other. Sometimesa brass conductor i s fi xed at the back of the rubber for theaccumulation of negative e lectricity .

T he Cylinder is Of different sizes, but in all cases i t i ssimilarly blown

,so that there may be proj ections at each end.

Securely fastened to these by means of cement are woodencaps which carry the axles

,and rest in two wooden supports

which are firmly fi xed into the wooden base . One axle i slonger than the other, and to this longer one the handle forturning the cylinder is fixed .

T he Prime Conductor consists of a hollow cylinder of

brass or t in , or it may be made of wood coated smoothlywith tin-fo il . It i s placed about the same height as the

F IG . 46.

F IG . 45 .

Comb or Rake .

Cyl inder Mach ine .

rubber, but on the opposite s ide of the cylinder, and is supported on a glass rod coated with shellac varnish . A row

of points proj ects from the portion- of the prime conductorfacing the cylinder. T hese are sometimes termed the combor rake

,and are about inch long, and brought within

about 3 inch from the surface of the cylinder.Both rubber and prime conductor are regulated by a

foot,which moves in a groove and is secured by a screw .

A knob is generally fixed to the prime conductor, when i t i slong in proportion to the width . Great care should be takenthat no po ints or sharp edges form a portion of the prime

T ex t-B oob of Frictional E lectricity.

conductor,except those turned towards the cylinder , other

wise the electrici ty will disappear as rapidly as it i s produced.

Charging the CylinderMachine.

—T his machine may becharged either positively or negatively. I ts usual charge ispos itive

,which is accumulated on the prime conductor ; but

when it i s charged negatively the negative i s accumulated ona conductor, which i s insulated and attached to the rubber.

Positive Charge — T o secure thi s the rubber must firstbe put into earth contact, and the prime conductor be insulated . On turning the cylinder the glass becomes chargedpos itively by the friction with the rubber, and the latternegatively, in accordance wi th the theory adopted in explaining the charging of the glass rod with the silk rubber (p.

T he rubber, however, being in earth contact, is enabled tointerchange a certain amount of free negative electricity with

the neutral of the earth, and thus to keep its units of electricity constant, and to decompose a larger quantity of thee lectricity of the cylinder. T he e xcited cylinder passes fromthe rubber

,charged with positive electricity

,towards the

prime conductor,the flap of o i led silk preventing its be ing

acted upon by the atmosphere . As i t approach es the prime

F I G , 47 ,conductor in a high state of

positive charge, i t acts by induction on the prime conductor

,

attracting the negative but re

pelling the posit ive . T he row

of po ints of the prime conduc

tor,facing the cylinder

,permits

negat ive to rush out against thecylinder and posi tive to pass in

Effect of turning C ylinden from it, thus securing equal unitsof e lectricity in conductor and cylinder, but in an alteredcondition . T he prime conductor

,which was previously

neutral, becomes rapidly and highly charged with positive,while the cylinder is as rapidly neutralised. It continues inth i s neutral state as i t passes from the prime conductor to

50 T ex t-Boob of Frictiona l E lectricity.

the rubber. T he free negative i s unable to interchangew i th positive from the earth

,and so i t accumulates with

increasing tension about the rubber,or the conductor

attached to i t . T he cylinder then passes to the prime conductor highly charged with positive . Here by inductionthrough the po ints

,and by means of the earth connection

,

there i s an in terchange of electricity, positive escapes fromthe cylinder to the earth , and negative passes from the earthto the cylinder. T his neutralises the posi tive charge of the

cylinder,which again passes to the rubber to yield up an

equal quantity of negative for a similar amount of pos itiveas at first.

T he amount of charge may be ascertained, as in theprevious case, by attaching a Quadrant E lectrome ter to theconductor at the back of the rubber. T he tension is dependent on the state of the atmosphere as regards moisture atthe time of the experiment .

Methods of showing T ension.

T he tension of the electrici ty of the prime conductor, or

the force with which a discharge takes place towards an

obj ect near it, is easily determined by bringing the knuckle

of the finger to the prime conductor. If the tension behigh

, the spark will be long, loud, sharp, and stinging ; but,i f i t be low ,

the spark,if any, will be feeble in all respects,

and scarcely perceptible . A rounded knob at the end of ashort rod of brass is the best means for testing this for then

the operator does not feel the pricking or stinging of the

high tension sparks, as they strike the knob held in the hand .

T he tips of the fingers when presented to the conductorreduce the tension of the discharge ; and if a sharp pointbe used no spark can be drawn from the prime conductor,and the pith-ball of the E lectrometer hangs vertically, as ifno charge had been communicated to i t. T he sharper thepoint, th e more successful is this experiment .

H enley’

s Quadrant E lectrometer.

Henley’

s Quadrant E lectrometer.

T his i s an apparatus which i s used for e xhibiting the

tension of the e lectricity of a charged body . It consists ofa brass rod about 6 inches long

,

“

arranged at the bottom to

fi t into small holes , which are sometimes left in electricalapparatus for that purpose, while the top terminates in asmall knob (fig.

T o this rod i s attached a semicircle of ivory, wood, orcardboard, graduated into degrees and from its centre issuspended a light wooden rod, having a smallpith-ball attached to the end of i t. In itsordinary condition the pith-ball hangs verti

cally downwards . When the brass rod of the

E lectrometer is attached to an electrified con

ductor, i t immediately becomes charged withsimilar electrici ty. T he pith-ball

,resting

against the rod, i s s imilarly charged, and immediately repelledf

‘

T he force of repulsionis measured by the number of degrees

,shown

on the semicircle by the rod to which the pithball i s attached

,and this angle i s an indication

H enley.

S Quadrant

of the tens ion of the free electrici ty in the body.E lectrometer

T he gradual loss of e lectricity into the atmosphere ,after the prime conductor has been charged, is clearly seenby placing an E lectrometer on the conductor, charging itwith electricity of high tension, and then watching the

gradual fall of the pith-ball,as théCharge is being reduced .

T he effect of presenting the knuckle, or a po int held in the

hand, to a highly charged conductor i s also clearly seen by

the sudden fall of the pith-ball to i ts vertical posi tion .

T he Plate Machine.

T he Plate Machine has the same essential parts as the

cylinder,the only difference being that the cylinder is re

placed by a circular glass plate having a thickness of aboutE 2

F I G . 48.

52

i in . T he rubbers are securely fi xed on both sides of theplate over each other

,one pair at the bottom , and the other at

th e top of i t and the branches of the prime conductor carrying the po ints or comb

,

sometimes termed the rake,

are situated midway between the rubbers . Insome machines there i s arow of po ints on each sideof the plate but in othersonly one row against theside of the plate nearestthe prime conductor .F rom the rubbers aboveand below on both sides of

the plate, o iled-silk flapsextend almos t to the

po ints of the prime conductor,to re tain

”

the positive chargeas in the cylinder machine . T he prime conductor is insulated, and the rubbers are in suitable earth connection bymeans of the wooden supports . Positive electrici ty i s thuseasily and rapidly developed on the prime conductor. F or

experimental purposes the plate machine is be tter than thecylinder . An ebonite plate i s sometimes used instead of aglass plate, and then the prime conductor i s charged withnegative electricity.

A metal plate might be used instead of e ither, provided i tbe well insulated from the spindle

,but as the generated

electricity would spread over the who le surface of the plate ,for the metal i s a good conductor, instead of remaining on

the portion of i t where i t was excited, the electricity from

such an arrangement would have very low tension .

FIG . 49.

P late M ach ine .

Method of Charging.

T his i s exactly similar to the charging of the cylinder,and need not be repeated here , if that be understood, see

Winter’s M acnine.

p . 48 . T he friction of the rubbers and plateelectricit ies of each, and the plate becomes chT his positively excited

“

plate causes posi tive to accumu

late on the prime conductor . T he positive charge can onlyacquire a certain tension

,for the atmosphere acts upon it in

such a way that when that tension is reached,the air dis

charges as fast as the machine excites it. A machinethoroughly insulated, as, for example, placed on a table witha glass top, may have i ts prime conductor easily charged byallowing a sharp po int, or a series of sharp po ints, to proj ectfrom the back of the rubber. T he free negative of therubber is thus enabled to escape into the atmosphere

,and an

equal quantity of pos it ive to enter through the points fromthe atmosphere

,thus securing the neutral condition of the

rubber which is necessary to continue the exci tement of theplate .

Winter’s Machine.

T he Winter Machine i s a modification of the PlateMachine

,and waS deve loped by

Winter,of V ienna . It differs from

it in having only one pair of rubbers instead of two, and the primeconductor spherical in form insteadof cylindrical. T o this sphericalconductor two rings are attachedwhich are placed on each side of

the plate. T he port ion Of therings towards the plate is grooved,and lined with metal

,from which

a series of po ints extend towardsthe plate, which assis t in chargingthe prime conductor as in the

o ther cases,or a ring of th in metal

i s fi xed in th e middle of the

groove,and has its edge turned towards the plate . A

ring, having a diameter of 3 or'

4feet, is sometimes fixed

F IG . 5 0 .

Win ter’s Mach ine .

54 T ex t-Boob of F rictiona l E lectricity.

into the top of the sphere to increase the surface of theprime conductor (fig . T his enables much longer sparksto be obtained from thismachine than from the ordinary ones .T he exterior of the ring is of pol i shed mahogany

,which

covers a very stout brass wire . I t is necessary that the ringand its connection should be perfectly smooth in all its parts

,

otherwise the electricity will escape,and the tension become

rapidly reduced .

T his machine i s charged similarly"to the others,and

like them may be charged e ither posi tive ly or negatively byconnecting the rubber or the conductor with the earth

,and

insulating the part required to be charged .

T heories of Charging the Prime Conductor.

T he theory which has been given to account for thecharging of the prime conductor of the E lectrical Machinei s what is termed T lie A ddition and Subtraction l eozy,

’ andis the most sati sfactory explanation if the T w o F luid

T beory’of e lectricity be adopted. T wo other theories have

been advanced,both of which are worthy of attention

,they

area . T he Addition T heory .

b. T he Subtraction T heory .

a . T he A ddition T heory supposes that the excess or

pos itive, which charges the prime conductor, i s due to thepassage of electricity from the cylinder or plate through thepo ints to the conductor. It thus receives more electricitythan i t would have in its neutral condition and is said to

be posit ively charged . T his explanation is most consistent'

with th e ‘ O ne F luid T heory ’of electrici ty .

b. The Subtraction T heory— T he subtraction theory

assumes,that

,when the positively excited cylinder or plate

approaches the prime conductor, it acts upon it inductively,attracting the negative

,but repelling the positive . T he

position of the po ints, facing the cylinder or plate, permits

the rapid and continuous'

discharge of negative electricity

E xperiments w it/t E lectrical M ac/zine.

from the prime conductor to the e xcited glass T his i ssufficient In quanti ty to neutralise it . T he posit ive electrici ty,remaining in the prime conductor, as the negative i s beingwithdrawn, increases in tension , until i t becomes so great asto interchange with the adjacent atmosphere

,and so main

tain the conductor uniformly charged . T he supporters ofthis theory contend that no positive enters the prime con

ductor, but that it i s entire ly charged by the withdrawal ofthe negative through the po ints . T he conductor would

thus be always positive,and the rake turned towards the

glass negative.

E xperiments with the E lectrical Machine.

Wel l warm and wipe the machine with a clean warm duster befo reuse . Connec t the rubber with the earth, and , after having regulatedthe pressure of the rubber agains t th e cylinder or plate , turn the handle .

Bring the knuckle of the finger to the prime conductor of th e machine— no tice spark . This is long, sharp , and stinging , when the weatheris d ry and frosty but feeble when the air is damp .

Ho ld a brass rod terminating in a knob to the charged prime con

ductor, bet ter sparks are produced . Present a po in t towards it , and nospark will be noticeable .

Fix Henley’s Quadrant E lectrome ter to the prime conductor not icerise of pith-ball , as the machine is turned . Take spark from primeconductor with knuckle or knob at the end of brass rod , the pith-ballfalls . Present a poin t to prime conductor

,and turn machine , the pith

ball remains at rest remove the point away from ~ the influence of the

conductor , the pith-ball rises ; bring the po int again towards it , the ballimmediately falls .

C harge the prime conductor, and not ice th e he ight to which the

pith-ball rises . Cease turning th e machine,and mark the gradual fall

O f the pith-ball , without any visible influence affecting it . T he quickness of the fall indicates th e invisible influences which are tending toreduce the tension of the prime conducto r.Make a tasse l of tissue paper , and fasten i t to short rod of deal

attach i t to the prime conductor,th e strips will repe l one another , as

the prime conductor becomes charged . H o ld a pointed me tallic rodin the hand , cover th e po in t with the fore-finger, and present i t torepel led strips— no tice effec t ; remove th e finger

— contrast result . A

head O f hair is some times used in a similar way.

T ex t-B oole of Frictiona l E lectricity.

Attach a po int to prime conductor,and turn th e machine no spark

can be obtained . Fix E lectrometer on prime conductor,there will be

no rise of the pith-ball , while th e po in t is attached . Fasten tassel asbefore , there will be l i tt le or no repulsion of the strips . This arisesfrom the reduction of the tension of th e e lectricity of the prime con

ductor by the rapid interchange of e lectricity through the point .

Various o ther experiments may be performed with the

E lectrical Machine, if the necessary apparatus be at handsuch as

T be E lectrica l Wh irl — This consists of an arrangement similar tothat represented in th e diagram fig. 5 1 . Four pieces of brass wire are

F ro . 5 1 ,bent at right angles , and , terminating at a , b, c, d , insharp points , are fixed at right angles to one another ina circular piece of brass

,e. This central portion has a

hole in th e middle of i ts lower face to rest on a pivot,

that it may move with the slightest amount of friction .

T he support for this whirl is a short brass rod,ending

in a sharp point . On turning the machine the whirlro tates in a direction opposite to that to which the

po ints are turned . T he speed of rotation is greatestwhen th e tension is highe st , and least when it is lowest .

E lectrica l I nclined P lane.-Ano ther striking experiment to illustrate

the effect of points is afforded by what is known as the E lectricalInclined P lane .

A rectang ular wooden base has four glass rods fixed into i t , tw o atthe front , and tw o at the back . T he front rods are shorter than the

P ro . 5 2 .

back ones , as represented in the diagram fig.

52 . T o the top of each rod a brass ball isat tached , and tw o paralle l wires , fastened to

th e knobs, ex tend from each front to each backrod . T w o thin wires , a , b, c

,d, cross each o ther

at right angles,and th e ends are also bent at

right angles , and terminate in sharp po ints .T he po in ts a and b are horizontal , but face in O pposite directions while c and d are vertical andpoin t respectively upwards and downwards .

On connecting th e arrangement with th e e lectrical machine by meansof a wire or chain

,attached to one of th e knobs , and charging th e

prime conducto r, th e wires become charged , and th e e lectricity escaping

through th e po ints causes th e wire c d to ro tate on a b as an axis, and

a b at the same t ime to rol l up the inclined plane.

ElectricWh irl .

E lectric Inc l ined P lane

T ex t-Boob of Frictional E lectricity .

ments are explained in the same way as the pi th-bal l movements in theprevious experiment . T he figures may also be used in the same glassvessel as th e pith-bal ls are placed in .

E lectrified Water. T o perform this experiment a small brassbucke t is used , which has a tiny ho le in th e bo t tom . T he bucke t is

part ly fi lled with water , which drips drop by dropfrom th e small ho le beneath . On hanging it to the

prime conducto r of the machine and charging it, the

drops succeed each o ther so rapidly from the re

pulsion which takes place,be tween them

,because

they are charged with similar e lectrici ty , that theyappear to flow in a continuous stream . Ano thervery striking experiment to i l lustrate the mutual repulsion of the particles of a body may be shown inthe fo llowing manner . Fi ll a small glass funnel ,having an opening in the stem abou t3th of an inch indiame ter , with fine si lver sand . I f the sand be not

e lectrified , i t w il l fall to the ground in a continuous straigth stream ;but , if a wire or chain be brought from th e prime conducto r into thesand , as soon as the machine is turned the sand particles forming thestream fly asunder , and spread out from the repulsion which takesplace be tween them .

H ead of Hair.—A wooden head

,having long coarse hair, is at

tached to the prime conductor of an e lec trical machine by means of abrass rod . As th e prime conductor becomes charged

,the hair rises,

and spreads out in consequence of th e repulsion,which takes place

be tween th e separate hairs, because they are charged with the samekind of e lectricity . A tasse l made of t issue paper

,and fastened to a

rod of wood , or to a thick brass wire , is somet imes used to il lustrateth e same principle .

T be H uman B ody I nsu lated — A lad standing on an insulatings tool , e i ther ho lding the prime conducto r by the hand , or con

nected with i t by means of a wire or chain,is for th e t ime being, as

i t were , a port ion of i t , and all -the phenomena exhibited by the

charged prime conductor may be also obtained from th e insulatedcharged person . Sparks may be drawn from parts of his body, con

ductors may be charged from him , and bodies in different e lectricalconditions may be attracted in some cases , and repe lled in o thers andif ano ther person , standing on th e ground , ho lds his hand over thehead of the charged person , th e hair will immediate ly rise towards thehand . I f a brass rod , terminated in a knob, be he ld close to a gas jet,through which th e gas is escaping

, the gas may be l i t bymeans of thedischarge from the human body.

FIG . 5 5 .

Electrified Water.

C ondensation of E lectricity .

F IG. 5 6 .

Sparkfrom Human Body .

10. Th e L eyden J ar and similar arrangements.

Under thi s divis ion of the subj ect we shall deal with

'what is known as the condensation o f electricity . T hisis effected by using two thin metal or metallic plates , asplates Of brass

,or tin-fo il, which are separated by some in

sulating substance, generally glass coated with shellac varnish,but sometimes ebonite

,or gutta-percha is used . O ne of

these plates is brought near to, or into contact with , the

prime conductor of the electrical machine, while the other

the second earth-connected plate.

T ex t-Boon of Frictional E lectricity.

T he accompanying diagram will explain the relation of

the parts of the condenser to each o ther,and its posi tion

with regard to the prime conductor of theelectical machine .

A represents the prime conductor,B

i s th e co llecting or accumulating plate,

and D the condensing plate, for i t i sfrom its action that e lectricity of a highertens ion is accumulated on B . c i s theinsulating substance

,separating the two

metal plates . T his is usually termed thedielectric

,because the electricities act

upon one another through i ts substance . E is the earthconnect ing wire .

T he three most common forms of electrical condensers

FIG . 5 7 .

E lectrical Condenser.

a . E pinus’

s C ondenser.b. F ranklin ’s Pane.

c. T he L eyden Jar.

a . E pinus’

s Condenser.

T his consists of two brass discs,

‘

separated by a dielectric,which is generally glass coated with shellac varnish . T he

discs are fastened to brass rods,which terminate in glass

handles for adjustment . T hese brass rods are supported byrods of glass, which terminate in a brass cap , fIirnish ed witha suitable arrangement for the brass rods to move freelythrough , so that they may be brought near to or withdrawnfrom the dielectric.

M etbod of cbargzng.-T he two plates are brought close

to , and on e ither side of,the dielectric

,and the strongest

charges are obtained when the plates are as near as theycan be brought to i t .When suitably arranged

,one plate is brought within

striking distance of,or connected by a wire or chain with,

the prime conductorof the electrical machine, and the other

Epinus’

s C ondenser.

i s connected with the earth . As the machine is turned, the

collecting plate becomes positively charged,and the con

densing plate negatively. T he co llecting plate i s pos itively charged from the prrme conductor by its attractingsome of the negative from the disc

,and communicating an

F IG . 5 8 .

E pinu s’

s Condens er.

equal quantity of posit ive to I t . T his positive charge,act

ing through the dielectric,decomposes the electricity of th e

condensing plate,attracts the negative 'but repels the positive .

T he repelled positive passes to the earth through the wire,while an equal quanti ty of negative is attracted from theearth to the

'

condensing plate. T his accumulated negativereacts through the dielectric on the opposite disc attractingthe posit ive but repelling more of—the negative . T he ten

sion of the two electrici ties is thus greatly increased, andthey are consequently drawn strongly towards one anotherthrough the substance of the dielectric .

If this material be too thin , or not uniform in i ts character throughout

,th e two electric i ties sometimes pierce the

dielectric,and thus neutralise one another, producing what

is termed a disruptive discbarge, and thus spo iling the arrangement .

T ext-Boob of Frictional E lectricity.

At other t imes the electricities acquire such density and "

tension at the edges of the discs,that they recombine over

the top, bottom,or sides of the dielectric sheet according

to the easiest passage afforded them . T his forms what isknown as the spontaneous disc/large. Bdth of these areirregular discharges .M et/sods of disclzarging. T he two methods O f discharging

a . T he slow discharge .

b. T he instantaneous discharge.a . T be slow discharge i s produced by touching the collect

ing plate with some conducting substance,and removing

the smal l amount of free po si tive, which i s always foundthere

,when the condenser i s charged . T he removal of this

free positive l iberates a certain amount of negative from the

condensing plate . T his may also be removed by touchingthat plate with the finger

, or some other good conductor .T he withdrawal of this again lib erates a small quanti ty of

positive from the first plate,and thus regularly

,and gradually,

this alternate discharge proceeds , until, after a very largenumber of discharges have been taken from each in succession

,the discs become thoroughly neutral ised .

T his discharge from one plate,and simultaneous libera

tion of electricity from the other,may be exhibi ted by

attaching small quadrant electrometers to the supports ,which are in metall ic connection with the discs the fall ofthe pith-ball in one case indicates the withdrawal from one ,

while the ri se of the oppos ite electrometer exhibits the

l iberation of electricity from the other plate .

b. l e instantaneous discbarge takes place when bothdiscs are connected by some conductor, or when a con

ductor,which i s in contact w ith the condensing plate i s

brought to the collecting plate to permit of the

e lectrici ti es he air space between the plate and theconductor

"

.

”“T he recombination of the two electricities i s

announced by a sharp crack,and a more or less brill iant spark .

T ne Discnarging R od.

T bc D isc/barging P od — T he apparatus,which is gene

rally used for discharging condensers , i s known by the nameof the discharging rod ; and is represented in diagram ,

fig . 5 9.

I t consists of two equal pieces ofbent brass wire,connected

by a hinge. At the end of each piece of wire is a smallbrass knob and this arrangement is fastenedby means of a brass cap to a glass handle

for the purpose of insulation . When one

knob is brought into contact with the con

densing plate, and the other near enough to

the co llecting plate, a spark is seen , and areport i s heard, which is more brilliant andlouder as the tension of the electricity accumulated on the discs i s greater. T hesephenomena arise from the recombination of

the two electricities of the discs through theconducting medium provided by the wire .

T he condenser may, ,be also instantaneously

discharged by connecting the two discs wi th the two handsin a similar manner to that adopted with th e dischargingrod, firs t touching the condensing p late, and while one

hand rests on it,bringing the o ther to the co llecting plate .

T he discharge then takes place through the operator’s body,and a shock feeble or severe is felt according to the size of

the condenser, and the tension of the charge . Severalpersons may experience this shock simultaneously if theyhold each other’s hand tightly

,and th e person at each end

of the chain touch the plates in the order previously de

scribed. T he heavier the charge is, the more severe will be

the shock . T hree things affect the strength of the chargeOf a condenser ( 1 ) the s ize of the plates (2 ) the substanceOf the dielectric (3) the nearness of the plates .

L arge plates enable a greater quantity of electrici ty to bedecomposed and condensed than small ones . T he charge

i s higher for equal thicknesses of dielectrics according to

F IG . 59.

Discharging R od .

64 T ex t-Boob of F rictional E lectricity.

the following order,air

,resin

,glass

,sulphur

,shellac

,india

rubber, gutta-percha .

As this condensation of electricity is due to the inductiveaction which take s place through the substance of the

dielectric,that

,which permits the electricity to act mos t

readily through it,is considered to have the big/zest specific

inductive capacity.

b. F ranklin’

s Plate or Pane.

T his i s the simplest form of electrical condenser,and

consists of a thin sheet of glass coated on each side withtin-fo il . T he fo i l should not reach the edge of the glass byabout two inches

,and the uncovered surface should be

coated'

with shellac varnish to prevent any escape Of electricity by means of the dewy deposit which frequently formson the uncoated glass . T his arrangement is also known by

the name Of the F ulminating P ane.

M et/cod of cbarging tbepane— T o charge F ranklin’s pane

the hand must support one sheet of tin-fo il,while the otherfo i l is presented to the primeconductor of the e lectrical machine . T he arrangement thenassumes the form of the con

denser— the tin-fo il resting on

the hand becoming the con

densing plate, and the sheet presented to the prime conductorth e co llecting plate . T he charge i saccumulated in a similar manner

Frankl in’

s Pane °

to that on the condenser, the

collecting plate i s charged posit ively and the condensingplate negatively. I t may also be charged by connecting theouter coating with the earth by a wire or chain

,and bringing

the collecting plate to the prime conductor.D iscbarging tlie P ane.

-T he pane may be dischargedlike the condenser, either slowly or instantaneously, by

FIG . 60 .

T ex t-Boob of Frictional E lectricity.

of th e col lect ing foil ; the pane is d ischarged , Shown by spark andreport .Show residual discharges of pane , as in condenser .Charge pane as at first, take shock wi th tw o hands , one on ei ther

side . Nex t let several persons j o in hands , and take the shock from thepane, when fully charged .

T o slzozv bind of E lectricity on eaclt side of pane.—Insulate pane ,

and at tach wire to condensing fo i l and gold-leaf e lectroscope . As soonas e lectrici ty accumulates on the co llecting foil , the leaves of th e e lectroscope diverge w ith posi t ive e lectricity. Test this by bringing fi rs ta glass rod , rubbed with silk near it , leaves sti l l further d iverge nex tsealing-wax, rubbed with flannel

,leaves collapse .

Obtain tw o shee ts of t in-fo i l of equal sizes, and a shee t of glassabout inches larger al l round . Coat the glass with she llac varnish ,and attach a silk handle to each end of i t by sealing-wax . B ring wireearth-connected to th e top of insulating stool

,lay fo il on wire , then

glass on fo il , and fo i l again on glass . Connect upper surface withmachine by means of chain or wire

,pass ano ther wire from it to e lectro

scope"Remove wire from machine by means of insulato r, l ift up glassplate gently by s ilk handles, th e leaves of the e lectroscope diverge withposi tive e lectrici ty.Charge pane

,test free e lectrici ty of co l lecting plate _

by bringing i tto e lectroscope . D ischarge this, then tes t free e lectrici ty of con

densing plate .

Another experiment consists in pasting bo th the fo ils on the glass .Fasten a tongue from th e lower round the edge of the glass . T he upperfoi l has also a long tongue , which is wrapped round a varnished glass rod .

By ro lling th e rod to and fro the distance be tween th e upper andlower fo ils is made lesse r or greater, and on working th e machine thespark increases in brilliancy and loudness, as one foi l is at a greaterdistance from the o ther.

c. T he L eyden J ar.

T he most important and useful form of the electricalcondenser is what is known as the L eyden Jar. I t was sonamed because the principles of i ts action were first determined by the philosophers of L eyden, a city in Ho lland.

In a rude form it was first discovered by "leist, aGerman clergyman

,A .D . 1 745 but its present construction

is due to D r. Watson .

H e was the first to coat the jar inside and outside with

T be L eyden f ar. 67

silver-fo i l . T he earliest jar was partly fi lled with water,and

held in the hand , a nail passed into the water and projectedbeyond the cork . T hiS

‘

was pre sented to the prime con

ductor of the e lectrical machine,and an interchange o f

e lectricities took place , between the inside and the primeconductor, and the outside and the earth

,and thus the j ar

was charged .

C onstruction of the L eyden J ar.

T he present L eyden jar consi sts of a wide-mouthedglass j ar or bottle, coated inside and outside with tin -fo il toabout two inches from its neck . T he mouthof the j ar i s closed by a piece of dried wood

or a large cork, through which passes a brassrod

,which must either rest on the inner coat

ing,or be brought into contact with i t by

means of a piece of wire or chain attached toth e rod . T he rod must be insulated from the

cork, and the end outside the j ar terminatei n a knob

,o therwise there will be a tendency

for the charge to escape into the air throughthe po inted end . I t is also advisable to givethe cork or wood and the uncovered neck of

the j ar a coating of she llac varnish for the purpose of betterinsulation . T he inside coating of the j ar i s generally usedfor the collecting plate, and the outside surface for the

condensing one . Under these conditions the‘

j ar would becharged positively ; but, i f the Ofiter coating be used as

the collecting plate, and the inside as the condensing plate ,the jar will be charged negatively .

T he j ar is considered to be charged with the kind of

electricity which is accumulated on the inside coating.

F IG . 6 1 .

Leyden Jar.

Method of charging the L eyden J ar.

a . P ositively— T o charge the jar positively, hold the

outer coating in the hand, or place i t in conducting c0n

F 2

T ex t-Boob of Frictiona l E lectricity.

nection with the earth by means of a wire or chain, 5 0

that there may be an easy means afforded for the e lectrici

ties to pass to and from the earth . T he knob must then bebrought e ither into contact, into conducting connection ,or within sparking distance of the prime conductor of the

electrical machine . As the machine i s turned,posi t ive

FIG . 62 .

Charg ing th e L eyden J ar.

electricity leaves the prIme conductor and passes into thejar, while an equal quantity of negative passes from the j arto the prime conductor . T his causes a rapid accumulationof positive electrici ty on the inner surface of the j ar

,which

acts inductively through the dielectric,attracting the nega

tive electricity of'

the outer coating,but repell ing the posi

t ive . T his repelled posi tive interchanges with an equalquantity of negative from th e earth , and thus the negativecharge of the outer coating is considerably strengthened.

T his outer charge of negative e lectricity reacts through thedielectric on the inner coating, and by attracting the posit ive more strongly

,and liberating more negative

,causes

the positive charge accumulated on the inside of the jar toacquire much higher tension than the e lectricity of the

prime conductor,from which the charge i s obtained. A

th in glass jar can rece ive a h igher charge than a thicker oneof equal size. When

,however

,the glass is very thin

,the

jar i s l iable to be pierced, if i t be not . of uniform character

T ne L eyden far. 69

throughout, iby the opposite electrici ties penetrating the

die lectric, and producing what is known as a disruptivedisc/zarge, which spo i ls the j ar .

T he repulsion of positive from, and attraction of negativeto

,the outer coating, as posi t ive e lectric ity accumulates in

side the jar,may be exhibited by several s imple experi

mentsIf the j ar be placed on some insulating substance

,and

a wire be attached to a go ld-leaf electroscope, the leaves

inStantly diverge, as electricity interchanges between the

machine and jar .Again

,a lad standing on an insulating stool

,and holding

the outer coating in his hand, can attract and repel the pithball of the electric pendulum ; by holding his hand over thedisc or ball of the electroscope

,he can cause the leaves to

diverge , and i f necessary charge i t by presenting his handnear a well-balanced lath which is free to move horizontally

,

he can,by attrac ting it

,draw it round and by bringing his

knuckle near another person , a spark will be seen to passbetween them . I f a wire attached to a jar s imilarly insulated be connected to one disc of E pinus

’

s condenser,and

the other disc be connected with the earth,the condenser

may be charged,and a series of L eyden jars may be simi

larly charged when suitably connected as in the cascadearrangeme nt (see p .

b. N egatively— I f the knob be held in the hand, or

otherwise put into conducting earth connection,and the

outer coating be brought into contact wi th, orwithin sparkingdistance of

,the charged prime conductor of an e lectrical

mach ine, in a similar manner to that previously described,pos itive electricity of high tension accumulates on the out

side, and negative on the inside. T he j ar is thus charged

negatively .

O tber results — I t i s generally stated that an insulatedj ar cannot be charged, but th i s should be modified to some

extent for the character of the insulation affects the amount

T ex t-Boole of Frictiona l E lectricity.

of the charge capable of be ing accumulated in the jar. Ifthe jar be hung from the knob by a si lk thread the insulation is most perfect

,and the indisposit ion to receive a charge

i s most complete . When i t stands on an insulating stool, afeeble charge may be imparted to the jar ; for the neutralelectricity of the wood of the top of th e stool will enable acertain interchange of e lectric itie s

,and

,consequentl y, a cer

tain amount o f positive and negative to become bound on

the interior and exterior coatings of the jar ; and thus aslight charge i s produced . A feeble charge , which undercertain conditions may be strengthened, i s communicated, i fthe j ar, while being charged, rest on a sheet of ebonite,resin , or o ther insulating substance . T he repelled positiveof the outerlcoating acts through the insulating sheet, andattracts negative from the earth to the other s ide of i t .T his produces a similar arrangement to the L eyden jar, or

forms with the jar a kind of compound condenser. F romthis cause the jar may be charged with electricity of a cer

tain low tension . In a short t ime,however, the tension of

the repelled pos itive of the outer coating,and the attracted

negative from the earth becomes so h igh , that a long sparkis perceived to pass across the surface of the insulatingsheet. T his discharge enables the jar to become more

highly charged than i t o therwise would be .

N o charge can be communicated if both the inner andouter coatings be in conducting earth-connection as underthese circumstances the posit ive electrici ty from the prime

conductor interchanges with an equal quantity of negative

from the earth by means of the connection with the innercoating without at all affecting the outer coating.

T o discharge the~

L eyden J ar.

T his may also be done , as was the case with E pinus’

s

condenser,either slowly or instantaneously .

a . Slow ly— T O discharge the j ar slowly it should be

placed on an insulating stool, or some insulating substance,

S low D is c/carge of L eyden f ar. 7 1

and be touched alternately commencing with the knob,

which i s in contact with the inner coating. A little of the

free pos it ive i s thus removed from the inside of the j ar thi sliberates a little negative from the outs ide, which is also free ,and may be removed by touching it with the hand

, or any

conductor in earth-connection . T h e removal of th i s setsfree ano ther small quantity of po si t ive from the insidecoating

,which on being discharged

,l iberates negative again

from the ‘Outer coating. T hese alternating resu lts take placeat

‘

each discharge of inner and outer coat ing, until, after aconsiderable number of discharges have been taken

,the

whole charge of the jar is neutralised .

T he slow discharge may be made an interesting experiment

,as shown in the accompanying diagram

,fig. 63.

T wo bells, d and e,are arranged on the same level

,one

being fi xed at the top of the L eyden jar,and the other on

the stand of an electric pendu F I G , 63,

lum,w hich i s in metall ic con

nection with the outer coating

of the jar. T he ball i s a lightcopper one, and is suspendedby a fibre of silk .

When the jar is sufficientlycharged

,and brought at a suit

able distance from the insulatedball

,the free e lectrici ty of the

inner coating attracts the copperball

,which strikes the bell‘ and

produces a ringing sound. T he

ball is instantly charged and re

pelled ? but it i s then attracted Experiment showing S low Dischargeby the negative bell on the stand of L e i den J an

of the pendulum . T his i t strikes , producing another ring ;

but at the same instant the ball is neutralised and charged

negatively by the contact, and repelled to the bell attached

to the inner coating of the j ar. T his again charges and

T ex t-Boole of F rictional E lectricity .

repels i t ; and similar results fol low, over and over again,until the jar is completely discharged . During this discharge the bells ring from the ball striking them alter

nately as long as the interchange takes place . T his ex :

periment i s mos t successful when th e j ar is large and the

air dry.

b. I nstantaneously.

— T he instantaneous discharge may beproduced either by using the discharging rod, or by connect

ing the outer and innercoating by the two

hands . In the formercase a similar planmust be adopted to

that used with the condenser

,viz . ,

the outeror condensing surfacemust first be touchedwith one end of the

rod,and

,while one

knob IS In contact withthis

,the o ther must be

brought near to the knob of the jar. A brilliant sparh and

sharp report, if the charge be high, announce the recom

bination of the two electrici ties, and what is known as thedischarge of thej

'

ar.

Should the jar be discharged by connecting the two

coat ings with the hands of a person,the outer coating

should be first touched, as before described and then the

knuckle of the finger be presented to the knob. T he spark_

and report are also heard in this case , but at the Sameinstant a shock more or less severe i s fel t by the experi

menter . A strong discharge may be felt by several personssimultaneously, providing they hold each o ther’s handst igh tly

,and the person at one end of the chain touches the

outer coating, while the one at the other end presents his or

her knuckle to the knob .

F IG. 64.

Discharge of Leyden J ar.

T ext-Boob of Frictional E lectricity.

conducting connection with the inner coating. T he apparatus in i ts complete form is represented by A .

F IG . 65 .

L eyden J ar w ith mov able coatings .

T he method of charging is s imilar to that of the

ordinary jar . When charged, it should be placed on aninsulating stool

,or a shee t of ebonite or vulcani te

, or on

some insulating substance, and the i nner coating shouldbe removed by means of an insulating rod

,or the fingers

pro tected by indiarubber tubes . A stick of sealing-wax orrod of ebonite will do well for this . T he glass may then

be removed with the unprotected fingers . When the partsare thus separated each coat ing may be discharged, and thedischarging rod brought to the two sides of the glass in asimilar manner to the method ofproducing the instantaneousdischarge of the L eyden jar.On restoring the parts to their former pos itions, a

brill iant spark Wi ll be O btained from the arrangement,

when the outer and inner coatings are connected by thedischarging rod or other conductor. T o secure the successof this experiment care must be taken in reconstructing thej ar. T he outer coating ,

should first be placed on someinsulating substance

,the glass next l ifted into i t with the

unprotected hand,and the inner coating finally restored by

the h elp of the same insulating rod .

L eyden B atteries, —A L eyden jar can only receive a

L eyden Batteries .

7 S

certain definite amoun t of charge,dependent on i ts size

,the

thickness of the glass, and the extent of the coated surface .

Other th ings being alike,the charge is always strongest

when the glass i s thin . A larger jar of s imilar th ickness to asmaller one will also rece ive a greater charge than thesmaller one . Hence powerful charges may be obtainedfrom large and thin j ars but these are expensive

,and very

liable to be pierced by the discharge of the two electricities

through the thin glass thus spo i l ing the j ars for furtheruse .

”

T hey are, therefore, not generally used for experi

F IG . 66 .

O rd inary form Of Leyden Battery.

mental purposes . Instead of large jars,a number of small

ones are connected together to produce these powerfuleffects . Such an arrangement is called a L eyden battery .

L eyden batteries are of two kindsa . T he Ordinary F orm .

b. T he C ascade Arrangement .a . T he O rdinary F orm — T he ordinary form of the

L eyden battery consists of several jars, so arranged that theirouter and inner coatings may be connected in such a manner

,

that the outer coatings may form one continuous outer

T ex t-Boob of Frictional E lectricity .

surface,and the inner coatings a continuous inner one. T his

i s secured by placing all th e j ars in a box,which i s l ined

with tin-fo i l,on which the outer coatings of the jars rest .

T he inner coatings are connected by means of brass rodsjoining the knobs together. T he external tin-fo i l i s con

nected with the handles of the box,and to one handle a

chain is attached to secure the necessary earth-contact .C harging and discharging

— T o charge this combination the inner coating must be connected with the primeconductor of the electrical machine by means of a chain or

wire,and the chain attached to the handle of the box must be

brought into perfect earth-contact . On turning the machinethe electricity is accumulated on the jars in a similar mannerto the s ingle jar, described in p . 68 . Several turns are requiredbefore the battery becomes charged with high tension electricity, and the larger and more numerous the j ars are, thelonger t ime i s required to charge the arrangement, and the

more powerful i s th e accumulated e lectricity stored in them .

T he force of the charge of such a battery is equal to that ofa large j ar

,whose inner and outer coatings are equal to the

sum of the inner and outer coatings of the j ars composing thebattery . I t i s also equal to as many times the charge of asingle j ar

,as there are equal jars connected together to form

the battery.

T he Universal D ischarger.-T o discharge a L eyden

battery an apparatus named the Universal D i scharger isrequired . I ts construction will beunderstood from the accompanying diagram (fig. A i s a smallivory or ebonite table

,B and c

are two movable brass rodsterminating in knobs

,whic h are

fastened to universal jo ints,and

these supported on glass legs .When a battery is to be discharged

,the knobs are brought

within striking distance above the table. One rod i s con

F I G . 67 .

Un iversa l D ischarger.

Cascade A rrangement. 7 7

nected by means of a chain or wire with the handle of thebox, and is thus in contact with the outer coating of the j ars

,

while the other rod i s connected similarly with one end of

the jo inted discharger. T he other end of this dischargingrod is brought near to

,or into contact with , the inner coat

ings of the j ars . A considerable spark is then seen to

pass across the insulating table . Should any object,rest ing

on

‘

the table between the knobs,resist the passage of the

electricity from knob to knob, certain visible effects are

produced, according to the nature of the resisting substance,

as the e lectricity forces its way through it.T hese are of the following character illuminating

,heat

ing,fracturing

,and sometimes magnetis ing effects. T hus a

lump of sugar, a lemon , or an egg would be brilliantlyilluminated, a thin platinum wire would be volatili sed bythe discharge from a powerful battery

,a piece of wood

struck in the direction of i t s fibre would be split asunder,and a steel needle across which the discharge had been sentwould be magneti sed. Great care is required in discharging large batteries

,or serious accidents may arise if the

discharge should pass through the human body .

b. T he C ascade A rrangement— T he cascade battery is

formed by connecting a series of L eyden jars in the follow

FIG . 68.

Cascade arrangement of L eyden Battery.

ing way — T he outer coating of the first j ar is connected by aw ire or chain with the inner coating of the second, the outercoating of the second with the inner coating of the th ird,

T ex t-Booh of Frictional E lectricity.

and so on throughout the series. T he last j ar must have itsouter coating in earth-connection . Any arrangement of

th e j ars which secures this result would form a cascadebattery .

C harging and discharging— B y this combination a

number of jars may be very nearly equally charged . Onbringing the knob of

C

A into contact with the prime conduc

tor of a machine charged posi tively, i t interchanges itsneutral e lectricity with it , rece iving posi t ive, and giving upnegative. In this way posit ive accumulates in the inside of

A . T his posit ive repels the posi tive from the outer coating“

of A , which passes to the inner coating of B . B returns anequal quantity of negative back to the outer coating of A .

T he charge of B acts similarly on c,and so forth

,until the

last j ar i s reached,when the repelled posit ive is driven to '

the earth,and negative drawn from it to the j ar. After a

series of this kind has been charged , if a conductor bebrought to the outer coating of the last j ar

,and then to

the inner coating of the first,the electricities will s imply

recombine and reduce the j ars to the neutral condition .

T o secure the discharge of this arrangement the connectionsmust be removed with some insulating substance, and the

knobs connected together by a chain, or by metallic rods, soas to have all the inner coatings in connection . T he outercoatings must also be in conducting connection, which maybe done by placing the jars on a metal plate . In this waythe combination is equivalent to a single large j ar, whosecoated surface would be equal to the sum of all the coatedsurfaces of the series of j ars . T o discharge this battery,place a conductor in contact with the outer coating of

e ither of the jars, and bring the o ther end of i t to one of

the knobs: T he universal discharger may be employed here

for e xhibiting e xperiments similar to those described inconnection with the ordinary L eyden battery.

On charging by cascade arrangement Profes sor C areyF oster remarks When an electric battery is charged by

E xperiments w ith L eyden jar.

cascade, each jar receives a lower charge than the one

which precedes it , and a higher charge than the followingone ; the charge of the second

‘

j ar is in fact only equalto what the first would be if the thickness of the glasswere doubled for the inductive action by which its chargeis produced takes place between two thicknesses of glassinstead of only one . Similarly the charge of the third jar isproduced by inductive action taking place through threeth icknesses of glass, and is therefore equal to what the firstjar Would receive if the glass were made three times asthick

,and so on of the others . ’

E xperiments w ith the L eyden f an — E xamine construction of j arand discharging rod charge j ar by contact with prime conductor, andwire or chain connection with the earth , and d ischarge i t instantaneously.

Charge again by connecting the inner coating with the prime conductorby means of a wire or chain , re taining a similar connection with th e

earth as in the last case , and discharge as befo re . Obtain anothercharge by ho lding the j ar in the hand

, and , bringing the knob withinsparking distance of th e prime conductor

,again discharge . Charge j ar

negative ly by ho lding kn ob in the hand,and bringing outer coating

within sparking distance of the machine .

Show slow discharge by placing jar on some insulating substance,and touching alternate ly the knob and the outer coating with the hand .

I f possible exhibi t slow discharge by be l l and ball arrangement .D ischarge a charged j ar instantaneously with di scharging rod . DO

so again by touching the outer coating and the knob with the tw ohands . Send charge through several persons join ing hands .P lace j ar on some insulating substance ; attach wire to e lectroscope

no tice divergence of leaves,as j ar is being charged remove wire con

nection test k ind of e lec tricity\with glass rod rubbed with silk ,

repuls ion , posit ive .

S tand a lad holding outer coating of j ar in hand on insulatingstool ; commence charging j ar pre sent knuckle to pith-ball of e lectricpendulum ; not ice attraction , charge, and repulsion .

L et lad bring his hand over the top of an e lectroscope notice divergence of leaves with free positive test as before touch e lectroscope whileunde r induction free posit ive disappears remove earth-connect ionwithdraw inducing hand the leaves diverge with negative e lectrici tytest with sealing-wax rubbed with flanne l ; no t ice greater repulsionsimilar e lectricity.

T ext-B ooh of F rictiona l E lectricity.

Balance lath , so that i t may be free to move horizontally let ladstanding on an insulated stool , and holding a Leyden j ar which is beingcharged in his hand , present the knuckle o f th e o the r hand to e itherend of th e lath , i t wil l be attracted , and may be drawn around .

L et another person present a knuckle to th e knuckle of the lad onthe s tool , and a spark may be drawn from him .

If the j ar stand on some insulating substance,and a wire

be led to one plate of E pinus’

s C ondenser,the o ther plate

being sufficiently near and in suitable earth-contact,the

condenser may be charged from the free pos it ive,which i s

repelled from the outer coating of the j ar that i s beingcharged . Show this, and discharge the condenser.A series of L eyden jars may be s imilarly charged in this

way,when suitably connected, as in the cascade arrange

ment (see p. Show this .

E xperiments .

—Charge Leyden battery,and discharge i t , using

universal discharger and discharging rod,through lump of sugar

,egg,

series of eggs in tube,ivory ball

,and lemon— no tice brilliant effect .

Ignite gunpowder by battery to do this a piece of w et String must beplaced in th e circuit .Show similar experiments with cascade arrangement .

T he Unit J ar.

T he unit j ar i s a small L eyden jar,which is useful and

necessary to measure the quantity of electricity accumulatedin a L eyden jar or battery . T here are two common forms,Harris ’s U nit Jar and L ane

’s E lectrometer . T he former i splaced between the prime conductor and the jar or batteryto be charged, and is connected with the inner coating ; butthe latter is away fromthe prime conductor, and attached tothe outer coating of i t.H arris

’

s Unitf an— T his consists of a glass tube, closedat one end, about 6 in . long and in . in diameter. L ike theordinary L eyden jar i t has metall ic surfaces inside and out

to within a short distance from the top of the tube . T hisportion i s coated with shellac varnish . Besides the knob at

T ex t-B ooh of F rictional E lectricity.

one end in a knob and at the other in a metallic ring,moves

freely through this . T he knob attached to the brass rod

faces the knob of the inner coating of the L eyden jar and,

F IG . 7 0 .

Lane ’

s Unit J ar or Electrometer.

as the brass rod i s movable through its support,the knobs

may be easily adjusted as to their distance from one another.T he outer coating of the unit jar must be in metallic connection by means of a wire or chain with the brass rod

,and

this coating should be also in good eafth -connection .

Before charging the L eyden jar it should be placed uponan insulating stool, and its outer coating put in conductingconnection with the inner coating of the unit jar. By thecascade principle the charging of the L eyden jar alsocharges th e unit j ar

,and, when the quantity of electricity

accumulated in the unit jar i s sufficient, a spark is seen topass between the two knobs. T his serves as the unit ofmeasurement for the quantity of electricity in the L eydenjar. A second spark announces that double the charge hasbeen received by the L eyden jar, and so on ; n sparksshowing that n t imes the quantity had been accumulated .

If from this arrangement a certain definite charge i s requiredfor a jar or battery the number of sparks must be countedfrom the unit jar, and, when these are equal, equal charges

‘

will have been communicated .

T he C ondens ing E lectroscope.

T he Condensing E lectroscope.

T he condensing electroscope, by means of a combinationof the principles of the electroscope and the electrical condenser, renders small and very delicate charges of electricityvis ible . In the e lectroscope portion of the apparatus thebrass rod connected with the gold leaves has a large brass

F IG . 7 1 . F I G . 7 2 .

C ondensinE E lectroscope .

disc attached to i t instead of a brass knob . Another disc

of the same size i s fastened to an insulating handle, so as to

be placed Upon,or removed from the o ther, as desired . T he

faces of the discs , which are to be brought into contact, arecoated with shellac varnish, which serves as an insulator, and

acts the part of the dielectric, as in the ordinary condenser.

T he plate attached to the electroscope is the one which

i s touched by the body to be tested, and is named the col

G 2

84 T ext-Boob of F rictional E lectricity.

lectingplate, while the other, which is brought upon it, i s the:

condensing plate. When the body is brought into contactwith the collecting plate, the condensing plate i s touch edwith the finger, or otherwise connected with the earth

,the

s imilar electricity is thus repelled, and the opposite attracted .

On removing the finger, and then the charged body, the twoopposite electricities of the collecting and condensing plateshold one another bound by their action upon each otherthrough the substance of the dielectric, and the leaves of theelectroscope are unaffected . When the condensing plate i sremoved by means of the insulating handle, the leaves of theelectroscope immediately diverge, from the free electricitywhich has been liberated from the co llecting plate by thew ithdrawal of the oppos ite electricity of the condensing

plate .

Sometimes two metal rods terminated in knobs form part

of the apparatus. T hese still further increase the delicacyOf the arrangement

,as the portions of the knobs turned

towards the gold leaves become charged by induction withopposite electricities

,and these will tend to produce a

further divergence of the leaves .T his apparatus was first constructed and used by Vol ta.

E xpem'

ments .-Show both forms of unit j ar if possible

,and test their

Charge tw o j ars with same number of sparks, and compare strengthO f charge by intensity of discharge .

E xhibit and explain parts of condensing electroscope— test lightlycharged bodies , as insulated me tal rod , or small proof plane , whenbrought into contact with a charged body.

11. Distribution of E lectricity on Conductors.

When a conductor i s charged with electricity,the charge

is distributed on the sufy‘ace of the conductor

,and its arrange

ment depends upon,

the shape of the conductor on whichit i s accumulated. T o exhibit this , conductors of differentshapes must be insulated

,by being placed on insulating

D is tribution of E lectricity .

supports, as glass rods coated with shellac varnish, guttapercha rods, sealing-wax rods, &c .

, or hung by silk strings .T he fo llowing bodies have been most thoroughly tested

the Sphere,the cylinder, the ellipso id, the disc, and the cube.

T his testing may be done in two w ays .1 . By bringing a proof plane into contact with different

parts of the body, a nd then noting the strength of its chargeby means of a delicate electroscope.

2 . By bringing the pi th-ball of the electric pendulumcharged with s imilar electricity near the charged body. Itwill then be seen to be repelled to different distances fromdifferent parts of the body, and will appear to be floating ina layer of electrici ty of varying depth or density .

If we examine a charged sphere by the second method,the similarly charged pith-ball will float around it at equal

F IG . 73. F IG . 74. F IG. 7 5 .

Charged Sphere . Charged Cy l inder. Charged E l lipso id .

distances in every part, thus proving that the sphere issurrounded by a layer or fi lm of electricity, which has thesame density or depth throughout _ (fig.

T his might also be shown by means of the proof plane

touching different parts of the sphere ; for the amount of

charge,as shown by its effect upon the electroscope, would

be found to be uniform in every part.

A cylinder w ith rounded ends, charged and similarlytested, would show greater depth at the ends, and lesser

about the sides of i t (fig.

T he charged ellipsoid shows greatest depth at the po inted

86 T ext-Booh of Frictional E lectricity.

end,less at the broad end, but least across the shorter

diameter (fig.

When a disc receives a charge it is accumulated with

great dens ity all around the edge of the disc,but in a very

thin layer on e i ther s ide of it. Such is the distribution on

the disc of the electrophorus (fig.

A charged cube has greatest density at the corners or

points of the cube, less on the edges, but least on the faces of i t.F rom the examination of such bodies as those described,

and others,i t has been established that the density of the

electricity of a charged body is greatest at points and sharpedges, less on the sides

, and least in hollow s or depressions .

In all cases the conductor must be away from the influenceof other conductors, or some disturbance of the regulararrangement may take place .

A n excited rod has electricity developed uniformly on theportion rubbed, and it accumulates there with increasingdensity if the rubbed body be an insulator

,as sealing-wax,

F I G . 76. F I G . 7 7 .

E xcited Glass R od.

Charged Disc . Exc ited M e tal R od in Insulating hand le .

ebonite, or glass (fig. Should i t be a conductor, as abrass rod held by some insulator

,i t spreads over th e whole

surface of the conductor (fig . 7 7 ) and is,consequently, of

less density,and lower tension .

Quantity, A rea, Density, T ension.

A charged sphere divides its charge equally with anotherinsulated, neutral sphere of equal s ize, and the half chargefrom either of these will again subdivide equally with anothers imilar sphere . T his will i llustrate the relation betweenarea and amount of charge . If the area be doubled, thequantity remaining the same, the dens ity or depth of the

S ubdivis ion of Chargel

of Spheres .

e lectricity is diminished by one-half. In diagrams (figs . 78 , 79,it will be perceived that

,when one unit of charge is spread

over two equal areas, the density in each becomes one-half

of what it was in the former case . T he half charge spread

over two equal areas has its density reduced to one-fourth of

the density of the original charge, e.g. Suppose a sphere A .

(fig . 78) has a charge Of e lectricity, represented by 48 units,F IG . 7 8 . F IG . 79. FIG . 80 .

Charged Sphere . Subd iv ided Charge . Further Subd iv is ion ofCharge .

and another equal but neutral sphere, B (fig. be broughtIn contact with i t and removed . T he second sphere wouldshare the charge equally with the first

,and the quantity of

e lectricity in each would then be represented by 24units .If again either of these be brought into contact with anotherequal but neutral sphere as 0 (fig. 80 ) the charge i s againequally shared

,and each sphere then contains 1 2 units .

Should this e xperiment be continued further,similar results

will be apparent,and the equal subdivis ion of the charge

maintained .

I f on the o ther hand the spheres be unequal,the distribu

tion will be in proportion to the area of the surface to be

covered. F or e xample,suppose a—s phere, F 1G . 8 1 .

having a certain area be charged withelectricity, which we will represent by 5 0

units, and that another sphere, having the

area be brought into contact with i t . T he

charge will then be distributed over the

increased surfaces in proportion to theirareas . N ow as one sphere i s of the area of the whole

increased surface,and the o ther 3, the smaller sphere will

Subd iv is ion of Charge .

T ex t-Booh of Frictional E lectricity .

take 3of the original charge,and the larger 3of It or the

quantity of electricity on each will be represented by 1 0 and

40 units respectively (fig .

In considering and comparing spheres i t should beborne in mind ‘ that the area of spheres varies as their

Another method of illustrating this principle, that altera

F I G . 82 .

Experiment show ing re lation between Area and Dens ity of Charge .

tion of area varies proportionately the density, i s afforded bythe apparatus represented in diagram (fig.

A metall ic cylinder i s insulated at one end,to which the

handle is attached,but is in metallic connection with a

quadrant electrometer, fixed on a brass knob, at the other.T he whole i s supported on glass legs

,and is usually fi xed on

a table, having an elliptical hole of sufficient size made in it .

T ext-Booh of F rictiona l E lectricity.

equally share the charge . Also if the hollow ball be firstcharged

,and the solid one be brought into contact with i t

,

exactly similar results are produced . T yndall states thatL e R oi and D’

A rcy showed that a hollow sphere accepted

the same charge when empty as when filled with mercurywhich augmented its weigh t s i x ty-fold .

’

A very striking and conclusive,but somewhat del icate,

experiment on this phase of the subj ect is exhibited by whatis known as B iot’s experiment.

T his experiment consists of covering an insulated chargedbrass or copper ball with two closely fi tting hemispherical

F IG . 83.

B iot’s Experiment.

metal cups,which are insulated by being fi xed to

handles . On separating the cups with very great care thee lectricity will then be found on the outer surfaces of thecups

,while the ball will show no trace of it s previous

electrical charge (fig.

Another experiment,which proves that the charge of a

hollow conductor i s found on the outside of it,i s to insulate

a hollow sphere,which has a circular hole about an inch in

‘

Dis tribution on Hollow Conductors . 9 1

diameter,for the insertion Of a small proof plane . On

charging the sphere, no el ectricity can be removed by theproof plane, if i t be brought into contact with the interior byinserti

‘

hg i t through the hole, which may be perceived bybringing the disc into contact with the gold-leaf electroscope ;while if the proof plane be made to touch the outside thedivergence of the leaves of the electroscope, as the disc is

'

brought near it,i s very apparent. Similar effects will be

Observed if a cylinder of wire gauze, a saucepan , a tin or

pewter pot, or even a man’s hat be insulated and used.

FIG . 84. F IG . 85 .

H o l low B ra ss Ba l l charged . Faraday ’s Bu tterfly N e t.

F araday’

s B utterfly [Vet (fig. 85 ) i s another experimentwhich conclusively illustrates the same principle. It consistsof a conical gauze bag about 9 in . long, and 4in . in diameter

at the base of the cone. T o the‘

apex of the cone two silkstrings are fastened

, one to the inside, and the other to the

outs ide . T he bag is secured to a wire or

'

narrow metallicstrip

,and supported on a glass rod coated with shellac

varnish . T he net, thus arranged, is capable of receiving an

92 T ex t-Boole of Frictiona l E lectricity.

e lectrical charge from any electrified body. When it ischarged

,if i t be tested with the proof plane and electroscope,

the charge will be found to be on the outside of the net.

T he conical bag is now turned inside out by means of thes ilk string

,attached to the interior of the bag the electricity

however, does not pass to the inside, but, when again tested,i s s ti ll found on the outside , as at first. And

,if the bag be

reversed several times by means of the silk strings, the elec

tricity always remains on the outs ide .

In order to s till further test the application of thisprinciple

,F araday constructed a cubical room

,having a

1 2 ft . side,which was built with laths spread over with paper

and wire gauze . T his be insulated from the earth by ropesof silk. In this chamber he performed the most delicate

experiments with e lectroscopes and electrometers, thoughthe ex terior of th e room was so highly charged withelectricity, that brill iant sparks were continuously drawnfrom it .Delicate electroscopes and electrometers may be pro

tected from the influence of highly charged bodies nearthem by being surrounded by a loosely fi tting covering ofgauze

,muslin, or t in-fo il . An electroscope

,placed in an

insulated bird-cage,or underneath an insulated gauze meat

cover, is unaffected when a charged body is brought near toi t . I t i s also not disturbed i f the cage or meat-coverreceives a high charge from an excited body

,for then the

charge accumulates on the outside of the hollow conductor,

and does not affect the electroscope within .

Another experiment in which this,

may be plainly éXh Ibited is described in Ganot’s ‘ N atural Philosophy ’ in thefollowing way —‘ T wo tin-plate rings, about 8 to 1 0 inches indiameter, are connected by

'

four thin vertical tin -plate strips ,a,b,c,d,and by twenty-four vertical wires , with each other

with the e xception of two , the latter are not shown so thatthe interior of the apparatus may be seen . T o each of thever t ical strips a paper band is fastened both inside and out,

94 T ex t-Booh of Frictiona l E lectricity.

body from the inside of the conductor,the leaves again

diverge with the electrici ty,which was previously held

bound on the ins ide of the hollow conductor by the presence and influence of the charged body . If this be tested

,

i t will be seen to be of the opposite character to that whichwas placed within the conductor

,and no electrici ty will be

found within the cup .

E xperiments .—Charge insulated sphere

,cylinder

,el lipsoid

,disc

,

and cube from excited rod,or

,bet ter , from an electrical machine .

T est density with proof plane and e lectroscope : no tice d ifferentd ivergence of leaves as different parts are tested ; also test each bysimilarly charged pith-bal l of electric pendulum

,and no te distance of

pith-ball from conductor in each case .

Insulate a sphere,e i ther by hanging it from a silk string

,or sup

porting it on a glass rod . Charge i t with e lectricity. Bring ano therinsulated and equal sphere into contact with i t . Test th e charges of

the tw o sphere s in both ways . B ring a third equal and insulatedsphere into contact with ei ther

,and test charge of each after contact

they will subdivide again equally. Show by means of apparatus (fig .

or by tin-foi l fixed to a glass rod , that when th e area is reduced byone-half the density of th e e lectrici ty is twice as great . Show also

,

generally, that when area is d iminished density increases , but whenarea is increased densi ty is d iminished . T o exhibit this attach wirefrom tin-fo il to e lectroscope , ro l l t in-fo i l on and unrol l i t from glass rod ,mark increase and decrease of th e divergence of the leaves as this i sdone . Obtain tw o equal me tallic spheres , one so l id and th e o therhol low ; insulate them ; charge th e sol id one , and bring th e ho l lowone into contact with i t compare the charges— equal . D ischarge each .

Nex t charge the hollow one , and bring th e so l id one into contact withi t compare charges again— equal . Same resul t apparent in bo th cases .Show B io t’s experiment, if possible ; prove e lectrici ty passes to th e

outside by removing i t on the cups . C harge hol low sphere , having acircular ho le in th e top ; pass proof plane through ho le to inside of

Sphere , touch e lectroscope no effect on leaves . Bring proof plane tothe outside , then remove to e lectroscope leaves d iverge .

C harge and tes t similarly cylinder of wire gauze , saucepan , tin , or

pewter pot, orman’s hat . N o charge will be found inside any of thesehollow conductors .

E xhibit , describe , and test Faraday’s butterfly net. This experimentrequires care .

C over e lectroscope loose ly with muslin or gauz e bag ; bring a

A ction of P oints . 95

charged body near the leaves are not disturbed .Place e lectroscope

in bird-cage or under meat-cover insulated , and bring an excited bodynear : no effect on leaves— e lectricity on the outside .

Prove this alsoby apparatus similar to that represented in diagram (fig.

Res t t in cup or o ther ho llow conductor on insulating stoo l or stand,

and insulated charged body within i t . °

C onnect outs ide with electroscope by wire : leaves d iverge . Test kind of e lectrici ty in leavessame as charged body within conductor touch outside of cup

,leaves

col lapse ; remove hand, then charged body from inside conductorleaves of e lec troscope again d iverge . Test kind of e lectrici ty— opposi tekind to that of body placed within , and the e lectricity which wason the outside . This is the elec trici ty which was bound on the insideO f the ho l low conductor.

13. A ction of Points.

Po ints and sharp edges,when they form part of a charged

conductor, permit a very rapid interchange of the electricitywith the surrounding atmosphere

,and thus quickly restore

the body to it s neutral condition .

Again,a po int

,which is in conducting connection with

the earth,brought near to

,and presented towards a charged

body,i s acted upon inductively by that body the opposite

electricity is attracted from the earth through the point, anddischarged on the body while electricity from the body isdischarged in equal quanti ty to the earth . T he sharper thepo int. the more perfect i s this discharge . T he effect of theaction of a po int may be exhibited by fix ing a quadrant

e lectrometer on the prime conductor of an electrical machine

,and letting the po int of a needle project from the

conductor. On turning the machine the electrometerceases to rise, for the conductor is almost neutralised by theaction of the po int. If the po int be removed, the electrometer rises to its usual position .

N ex t, i f the point be held in the hand, and presented to

the prime conductor, similar results follow, as in the pre

vions case .On covering the point with the finger the

e lectrometer again rises , but falls as soon as i t is uncovered .

Another good means of i llustrating this is by attaching a

T ex t Booh of F rictiona l E lectricity.

tassel of tissue paper by means of a deal rod to the primeconductor . As the prime conductor becomes charged

,the

tissue tassel i s also charged with the same kind of electrié

city,and the separate strips repel one another . On present

ing a point, those strips which are near it immediately collapse ,but diverge again , if the po int be covered. T he tension

of the e lectricity of the prime conductor i s considerablyreduced by a po int projecting from it

,or one in earth-con

nection directed towards it . Only a very feeble spark can be

obtained in e i ther case. T he latter arrangement resembles

the principle and action of the lightning-conductor,that

destroys th e tension of the cloud hovering above the building which the po inted conductor i s intended to protect .If the hand be held near the po int when the prime con

ductor i s charged, the electric wind , or rush of repelled airparticles

,i s fel t . T his i s known as the electrical aura . It will

act l ike a fine stream of wind on a candle flame broughtnear i t

,and will drive away a strip of tissue paper held

towards it .In th e prime conductors of electrical machines po ints

are used to permit o f the more rapid charging of the con

ductor by the interchange of e lectricity from the excitedglass to the conductor, and from the conductor to the glass .A po int

,or a series of po ints

,projecting from the rubber of

an electrical machine into the air, would enable th e primeconductor to become charged, even when th e machine wasperfectly insulated from the earth . T he po ints secure the

same necessary interchange with the atmosphere that a wire

or chain connection does with the earth .

An ordinary sewing needle , held in the hand,and the

po int of i t passed up and down over the different parts of ah ighly e xcited glass or ebonite rod, i s one of the bes t waysof thoroughly discharging it.

T he influence of a po int, when it i s attached to a con

ductor,may be illustrated by experiments s imilar to those

represented in th e diagrams (figs . 8 7 , In the first

T ex t-Book”of Frictional E lcctricity.

posit ively, whereas , if the po int remain attached to the con

ductor,the ball will be neutral ised from the interchange of

electrici ty with the surrounding atmosphere . T his method

o f charging a conductor i s the same in effect as charging bycontact, for th e conductor in this way is charged with thesame kind of electricity as that of the electrified body .

A pith-ball suspended by a fibre of s ilk , and having asharp po int projecting from it towards a charged body, i srepelle

‘d when b rought near it,from . ) the interchange of

electriciti es from the charged body to the po int, and fromthe po int to the charged body .

T he movements of the wire frame in the electrical whirl,and the electrical inclined plane described p. 5 6, are illustrations of the discharge of electrici ty through po ints .

E xperiments —L et a point proj ect from a conductor away frome lectrified body ; t ry to charge i t while th e point remains . Present apo int to conductor while i t i s under induct ion remove rod first , thenremove the point . Repeat th e experiment ; remove po int first , andthen remove rod or excited body . C ontrast effects in each case .

Attach po int , as po inted brass rod , or"needle , to prime conductor

of machine , and fix a quadrant e lectrome ter also to i t . Turn machine ,the presence of th e point prevents the rise of th e pith-ball . Removethe po int

,the pith-ball rises as usual . Present a point , held in th e

hand , to the charge prime conductor— the pith-bal l immediately falls ,and will not rise again until the po int be withdrawn al toge the r .No tice effect of covering po int with finger , and again uncovering i t onthe rise and fall of the e lectrome ter.Attach tasse l of tissue-paper to prime conductor , and test in similar

manner .Show decrease of tension of spark

, if po int be allowed to projectfrom , or be presented towards , the prime conductor.

E xhibi t e lectrical wind by its action on candle-flame , and on stripof tissue-paper , he ld near the po int .Insulate an e lectrical machine by placing it on a large warm shee t

of plate-glass ; attach a row of po ints to the rubber away from the

machine . C harge machine without earth contact .E xcite glass rod show difficul ty of discharging it . Take sewing

needle , move point up and down th e rod,and turn rod as this is done

perfect discharge . Test with e lectroscope . Fix point in insulated

E loctrical Disc/cargo. 99

conductor ; bring excited body to opposite part of conducto r ; whileunder induction remove point with insulator , conductor charged withopposite e lectrici ty . Restore po int , bring excited body towards point ;remove again with insulato r, conducto r charged with similar e lectricity.

Suspend pi th-bal l from fibre of raw silk attach a fine point to i t .Show repulsion of pith-ball as exc i ted body is brought towards thepo int .Show movements of electrical whirl and e lectrical inclined plane as

illustrating strikingly th e effect of the discharge of e lectricity throughpo in ts .

14. E lectrical Discharges.

Whenever there is an interchange of electricity of sufficient tension from a charged conductor with the induced

opposite electricity of an otherwise neutral body,or when

two oppos itely charged conductors interchange with one

another, two effects are always apparent,which announce

the discharge, viz . the spark and the report. T he spark isof exceedingly short duration , lasting only about aafia ‘

o" part

of a second, but it‘

is of great brilliancy . I t takes one of

three forms— s traight, s inuous, or zigzag,depending upon

its length . T he two latter are generally obtained from th e

prime conductor of an electrical machine under the mostfavourable circumstances, and these also are dependent to

s ome e xtent on the kind of machine , and the state of the

atmosphere . Dry frosty weather is the best for electrical

experiments .

T his interchange of electricity,which is known as the

E lectrical D i scharge, i s capable of producing a variety ofeffects, such as luminous, chemical, heating, mechanical,magnetic

,and physiological .

L uminous E ffects .

Besides the spark discharge there are others which are

known as the brush and glow discharges . T he former may bei llustrated by placing a small knob on the prime conductor

o f an e lectrical machine, or by sticking a pin into i t, and

H z

T ex t-Boole of Frictional E lectricity .

allow ing the discharge to take place in the dark from thehead . T he discharge from the pin’s head forms a divergentcone of l ight . T he brush discharge is sometimes vis iblefrom the masts of sh ips

,from the bayonets of soldiers or,

under very high electrical induction, even from the fingersof the uplifted hand . Under such conditions the dischargei s known as ‘St. E lmo

’

s fire.

’ If a very sharp po int, l ikethat of a needle, be used, and allowed to project from the

prime conductor, the discharge take s the form of a brill iantpo int of l ight, which is plainly noticeable in a dark room ,

and is known as the e lectric glow. T he spark discharge ispractically instantaneous but the brush and glow discharges are continuous . By suitable arrangements thisapparently instantaneous spark may be made to e xhibitbrill iantly illuminated designs . T o produce these, the design

,word

,or motto

,must be arranged on a sheet of glass

by means of small pieces of tin -fo il, pasted or gummed so

near each other as to be within easy sparking distance . T he

plate of glass is enclosed in a wooden frame . T o the frame,

and in metallic connection with the fo i l,a knob is fi xed at

the beginning of the design, and a ring is s imilarly connectedwith th e end of i t . T o th i s ring a chain i s attached

,for the

purpose of securing good earth connection . T he mostcommon illustrations of this are seen in the words ‘ F ire ’

and ‘ L ight,’ and what are known as the ‘ luminous spiral ’

and the ‘ luminous star ’ ; but sometimes e laborate floraldesigns and longer words are prepared for exhibiting thiseffect .

T he brilliancy of the spark i s considerably increased by

the discharge from a highly charged L eyden jar, or betterstill from a L eyden battery . T he battery discharge may bymeans of the U niversal D ischarger be sent through varioussubstances

,as egg, series of eggs, lemon, ivory ball , lump

of sugar,with varied illuminating effects . A L eyden jar

,

having its inside and outside coatings lined with lozengeshaped strips of tin-fo il

,or dotted over with tin-fo il spangles,

T ex t—B ook of Frictional E lectricity.

Chemical E ffects.

T he chemical effects of the electrical discharge are not sostrikingly exhibited, as they are by the passage of the Voltaiccurrent . High tension discharge sometime s decomposes

,

and at other times recombines compound bodies . T he

former is seen when a wire from the prime conductor isbrought into contact with a piece of blotting-paper, soakedin iodide of potassium and starch water

,wh ich has a wire

from the opposite end of i t connected with the earth . On

the paper nearest the prime conductor brown spots will benoticeable if blue litmus paper be used

,the spots will be

red . With regard to recombination, i f o xygen and hydrogenbe collected in their combining proportions— one of the

former to two of the latter— in a eudiometer, that i s, atube into which two platinum wires are fused, and a high

tension spark be passed through the mixture an e xplosionannounces the chemical combination of the two gases , and

a deposit of vapour i s the water thus produced .

Volta’s P istol i s constructed on this prinC 1ple . T wo

volumes of hydrogen and one of o xygen are placed in a

F IG. 90 .

Vo l ta’s P istol .

brass vessel,and the opening is closed with a cork . A

short rod, terminated at each end in a knob, p asses into the

vessel . On a spark passing from an electri c—

machine to the

H eating Efl eets .

outer knob, another discharge takes place within the vessel 5the gases instantly combine with a loud report

,and the heat

generated causes such an expansion of the vapour of waterthat the cork i s driven from the vessel with some amount offorce.

T he formation of ozone is another chemical effect of theelectrical discharge. I t i s produced when e lectricity of

high tension escapes through a series of po ints into the air,

and may be easily recognised by its peculiar odour . If apiece of blotting paper, which has been soaked in a solution

of starch water in which a crystal of iodide of potassium hasbeen dissolved, be held towards the head of a pin , proj ecting from th e

‘

prime conductor of an electrical machine, abrown colour is produced on th e damp paper, as the machine is turned . T his colour i s due to the action of ozone .

Heating E ffects .

By means of the electrical discharge . from the primeconductor of a good machine, or of a h igh ly charged

L eyden jar, ether, Lbisul F I G 9L

phide of carbon , and alco

hol may be ignited ; andwith a su itable arrangementthe gas may be li t . Avessel s imilar to that shownin fig. 91 is generally used

for experiments with liquids .T hey may

,however

,be

ignited in a small spoon

if i t b e heated sl ightlybefore the experiment istried . Ign iting Ether by sparkfrom Leyden Jar.

T o light the gas from the machine a person shouldstand on an insulating stool, and place the hand upon ,or be otherwise in metallic connection with the prime

104 T ext-B oon of Frictiona l E lectricity.

conductor . T his may be done by fastening a chain to theprime conductor, and holding the end of i t in the hand .

On presenting a brass conductor held in the o ther hand toa gas jet, through which the gas is escaping, the dischargeimmediately l ights the gas. A piece of ice

,held in the

hand,

tmay also be used, and so may the unprotectedfinger

,but a little care will be necessary in performing this

last experiment.If the L eyden jar be used for the experiment

,i t should

first be charged highly from the e lectrical machine ; thegas should then be turned on, and the knob of the j ar bebrought some l i ttle distance above the jet. T he ordinarydischarge of the j ar will now igni te the gas .

F rom the discharge of a L eyden battery a platinum wiremay be heated to a red heat

,a white heat, and sometimes

volatili sed. Gunpowder and gun-co tton may be explodedby introducing a wet string, or a wet piece of glass tubing inthe circui t

,so as to delay the discharge slightly.

Mechanical E ffects.

By the ordinary discharge of a L eyden jar a card may

be punctured,and the hole will show a fraying on both s ides

of the card,as if the discharge had passed both ’

ways fromthe middle of the card.

T he discharge from a L eyden battery will puncture asheet of glass of different thickness , according to the strengthof the battery

,and a strong battery will spli t a piece of

wood half an inch thick,i f the discharge s trike i t in the

direction of the fibre .

When a L eyden jar is very highly charged, and its glassi s not uniform th roughout, or when a jar has been high lycharged and allowed to stand for a time, the two e lec trici tiesin their endeavour to reach one another sometimes piercethe glass

,and produce what i s known as the disruptive

disc/l arge, thereby spo iling the jar.

T ex t—Boon of Frictional E lectricity.

Professor T yndall states that ‘ the Abbe N ollet formeda line of onehundred and eighty guardsmen

,and sent the

discharge through them all . ’ T he discharge on two occasions struck F ranklin senseless and from accidentallyrece iving the discharge of fifteen large L eyden jars

,T yndall

experienced peculiar optical disturbance or derangement .Describing thisv

accident he remarks ,‘ F or a sensible interval

of time l ife was absolutely blotted out,but there was no

trace of pain .

’H e regarded this as an experimental proof

that people killed by lightning suffer no pain .

L ichtenberg’

s F igures .

T hese peculiar coloured figures are produced on aninsulating substance— a cake of res in or ebonite is the best—by drawing patterns onthe surface of the insulator, with

the knob of a L eyden jar charged pos itively,or the knobs

of two L eyden jars— one charged positively, and the othernegatively. If only one jar be used, the surface of theinsulator should first be excited

,and then the lines traced

on i t with the opposite e lectrici ty of the knob ; but if two

oppositely charged jars are used , the e xcitement of the surface is unnecessary . After the tracings have been made,finely powdered red lead and flowers of sulphur

,mixed to

gether and placed in a fine muslin bag,should be sprinkled

over the surface of the insulator. By friction with one

another and the surface of the muslin, the red lead becomesposit ively electrified, and the flowers of sulphur negatively ;and accordingly the red lead arranges i tself along the negative tracings

,and the flowers of sulphur over the posit ive. In

this way very pretty designs may be produced . Should thewhole surface be exci ted negatively, and lines be traced on

i t with the knob of a positively charged jar, the whol e of

the surface will be red, varied by the yellow tracings of theflowers of sulphur but if both j ars trace l ines on an otherwise neutral surface

,the tracings will consist of red and

yellow patterns, varying somewhat in general characteristics .

Eject of H ea t on Disc/zarge.

E ffect of Heat on E lectrical Discharge.

An iron ball at a white heat cannot be charged with

electricity i f brought into contact with a charged body. Inthi s it resembles the indifference to magnetism of a whitehot iron ball. As the ball cools , i t first accepts a charge of

negative electrici ty, when it reaches a bright red heat andi t i s some time— not until i t becomes of a dul l red— that i tshows a disposition to accept a positive charge. Soon afterthis condition is reached i t receives a charge of either kindequally well . T hese statements may be proved by placinga white-hot iron ball on a gold leaf electroscope . At thattemperature the electroscope refuses to rece ive a charge .

As the ball cools the electroscope may be charged nega

tively, but the cooling must be continued for some timebefore the electroscope will accept a positive charge .

A similar effect i s apparent if an electroscope be discharged by the presence of a hot ball. If i t be at a whiteheat and earth-connected

,i t discharges a positively or nega

tively charged electroscope equally well . As the temperature falls to a red heat a negatively charged electroscopewould be discharged but a pos itively charged one wouldrequire a still lower fall of temperature . T his portion of

the subj ect has been carefully investigated by ProfessorGuthrie .

A ction of F lames .

F lames act like white-hot metallic bodies in the discharge of electricity

,and the cause of their action is due to

the motion of the air-particles arising from heat, and not,

Guthrie remarks,‘ that th e particles of the vapour or gas

act l ike a multitudinous collection of po ints affording egress

to electricity of one kind, and ingress to that of the other .’

T ex t-Boole of Frictional E lectricity.

E ffect of E vaporation on Discharge.

T his may be shown by placing a small metal lic ves se lcontaining water on the disc of a gold leaf electroscope . I fthe electroscope be negatively charged, it i s dischargedalmost instantly while

,if positively charged

,i t i s scarcely

altered in any way . T h i s,again

,i s similar to the action of

heat and flame on charged bodies .

I nduced Currents by Discharge .

Induced currents aris ing from a discharge of frictional

electrici ty are best i llustrated by means of flat spirals of

insulated copper wire . T hese should be brought clo setogether , and be parallel with one another . T he one whichi s to rece ive the spark from the electrical machine or

L eyden jar, and is to convey what is termed the primarycurrent, must have a knob at one end of the spiral

,and the

o ther end must be in good earth connection . T he secondspiral should terminate in two knobs

,which lie near each

o ther. When a spark passes to the firs t spiral,a current

proceeds th rough it to the earth . T his induces a current inthe second spiral, which passes th rough it in an oppos itedirection to that passing through the first

,and is named the

secondary current. I t i s instantaneous in its action,and is

shown by a spark passing between the two knobs at the

ends of the second spiral . By suitable arrangement a currentmay be made to pass through a third spiral, which is inducedby

,and is in the oppo site direction to, the secondary current .

T his is known as the tertiary current . E ach current ismomentary in its action , and is proved to exist by the

sparks which pass between the knobs that are near one

ano ther and attached to the terminals of the spirals.

T ext-Boon of Frictiona l E lectricity.

velocity, so does the resistance of the wire . Insulated sub s

marine wires , l ike ocean cables,reduce the velocity con

siderably.

A means of illustrating the great velocity of electricity isfurnished in the luminous spiral

,the luminous pane, and

experiments of a similar character . T he numerous breaksbetween the various small pieces of tin-foi l produce really asuccession of discharges

,as the discharge from the prime

conductor passes across them yet they take place so rapidlyafter one another that the eye fails to distinguish any difference as to the time of each ; consequently, the wholesuccession of sparks seems to appear together

,and the

design to be continuously illuminated .

E xperiments .— Show spark discharge— straight and zigz ag.

S t ick large p in into the prime conducto r of e lectrical machine ,notice discharge as the machine i s turned —brush discharge . Fixneedle into prime conductor so that point may proj ect , notice bri ll iantspeck of light at poin t— glow discharge .

Pass discharge through words Fire ’or Light

,

’ through device ,and luminous spiral .Charge Leyden j ar highly and discharge it— brilliant white spark .

C harge luminous Leyden j ar, no tice sparks as charging proceeds , andbrilliant effect as j ar is discharged . D ischarge highly charged Leydenj ar

,or Leyden bat tery through egg, series of eggs in glass tube, lemon,

ivory ball , and lump of sugar— observe luminous effec ts .E xhaust e lectric egg, and pass series of discharges rom prime con

ducto r of a good e lectrical machine through i t , the viole t glow will becontinuous . Admit air, no tice change to white intermit tent spark .

Place piece of gold , as sovere ign or half-sovereign, on prime con

ductor, take spark from it do the same with shilling or Sixpence ,

copper , zinc , or t in the sparks will be of various co lours .Soak blo t ting paper in a so lut ion of iodide of po tassium and starch

,

while damp bring i t to prime conductor, having head of pin projectingfrom it the formation of ozone co lours the damp paper .

Ignite e ther and alcoho l in a small spoon by discharge from machine ,and again from highly charged Leyden j ar. F or the igniting of e therth e spoon should be slightly warm ,

but for alcoho l made ho t ter. Abe t ter means of performing th e experiment is provided by the apparatusshown in p . 10 3.

E xperiments on E lectrica l Disc/cargo.

Light gas with brass rod , piece of ice , or finger , by stand ing on aninsulating stoo l , and rest ing one hand on th e prime conductor , or

ho lding a chain attached to i t . Do i t also by discharging a Leydenj ar j ust above a jet, through which the gas is escaping.

E xplode gunpowder and gun-cottonoon table of universal discharger.

E ach should be placed be tween the knobs , and a piece of w et stringinserted in the circui t to secure the success of the experiment .Rest a card , held in th e hand by a corner, on the knob of a Leyden

jar, discharge the j ar by bringing the lowe r knob to oute r coating,and

th e upper knob above th e card— small ho le pierced in card examinei ts character , frayed bo th ways .D ischarge Leyden bat tery through sheet of glass , and p iece of wood

in direction of fibre .

Lay a strip of t in-foi l on a shee t of varnished glass . Place a steelneedle or piece of s teel wire beneath the fo il , and ano ther piece abovei t . E ach should be at right angles to the foil . Send several sparksfrom a‘ highly charged L eyden j ar or battery along the t in-fo il . E xhibitmagne t isation and compare po larit ies .

C onnect a wire from prime conductor with one binding screw of

Astatic galvanome ter , and take a wire from th e o the r binding screw to

the earth , no t ice deflect ion of the need le as the machine is turned .

Charge Leyden j ar and discharge i t through class, joining handsphysiological effect .

E xcite cake of resin or ebonite , trace figures with knob of positive lycharged Leyden j ar. Sprinkle powdered lead and flowers of sulphurfrom fine gauz e bag over cake, the red lead adheres to the negative partof th e cake , and the flowers of sulphur fo l low th e tracings .Take a neutral insulating cake

,and trace figures with two Leyden

j ars,

one charged posi t ive ly and the o ther negative ly. Shakepowders over th e cake red lead fol lows the tracings of the negative j ar,and flowers of sulphur the positive . Contrast d ifferent results in eachcase .

H eat an iron ball to a white heat , insulate i t , bring i t near theprime conducto r of an e lectrical machine, attempt to charge i t , test i twith go ld leaf e lectroscope ; no effect . Test i t , when at a brightred heat, receives negative charge ; try posi tive , no effect. At lowertemperature , dull red , tes t positive again , now he ld . At lower temperature still, try bo th— each equally we l l re tained .

Bring white hot,earth-connected ball near elec troscope charged

positive ly, and ano ther similar ball near ano ther charged negative ly ,bo th wil l be discharged equally well . As th e temperature of the bal lfalls , negative i s first discharged , and , afterwards, posi tive .

T ex t-Boon of Frictiona l E lectricity.

Light lucifer match and hold it over the disc of an e lectroscope , thee lectroscope will be discharged if charged with e i ther positive or negative .

Soak some co t ton wool in alcohol , wrap wire around it , light thewool and hold the flame over th e disc of an e lectroscope , th e flamewill discharge th e e lectroscope .

Place water in a small metall ic dish on the disc of an e lectroscopecompare difference of effect when the e lectroscope i s posi t ively ornegatively charged .

Show induced currents with flat spirals . Ro tate disc divided intonumerous equal black and white sectors , no t iceuniformly grey appearance . Darken room and discharge highly charged Leyden j ar no ticesome black and white sectors are distinctly seen , some are not so

,due

to spark not béing absolute ly instantaneous . Procure a Newton’s disc , acircular card

,consist ing of th e seven prismatic colours arranged in their

due proportion , ro tate i t, and illuminate i t wi th a brilliant spark , thevarious colours are seen almost d istinctly.

15 . E lectrometers .

I t 1s frequently necessary to know not only the character,but also th e strength of the electricity of a charged body,and to ascertain the force or intensity of the action of one

charged body upon another, or upon a neutral body at adistance from it. T his is

,determined by the use of an

electrometerT he simplest forms of electrometers are

,the e lectric

pendulum,and the gold leaf electroscope . T hey,however

,

only roughly indicate the difference in intensity of charge.

T hey are properly e lectroscopes,enabling us to see that

electric ity is present, rather than electrometers,which pro

vide us with a means of determining accurately the relativestrength or intensity of charge .

T he chief electrometers in use at present are —Henley sQuadrant E lectrometer

, C oulomb’s T orsion Balance, the

Peltier E lectrometer, and Sir William T hompson ’s Quadran tE lectrometer.

T ex t Book of Frictiona l E lectricity .

upper disc has turned . F rom the centre of the upper disc a '

very fine silver wire hangs. T his passes through the middle

of the glass tube, and as far as the middle of the cylinder .T o the end of this wire a thin shellac rod i s fastened at itscentre p,

and carries at one end of i t a small disc of thinmetal fo i l n. T hrough the hole n ear the side of the vessel

,

a conducting rod passes , having pith-balls, r and 7n,at each

end of i t, one outs ide , and the other inside the glass cylinder .T he inner ball extends as far down into the vessel as th edisc attached to the glass rod i s s ituated

,so that the two

may be brought into contact with one another.A g raduated scale 0 passes around the cylinder at thi s

height,s o that the deflection of the disc from the pith-ball

may be easily measured . Although the calculation of the

electrical laws by the T ors ion Balance ’ belongs to the

advanced rather than to the elementary stage, yet an exami

nation of one of them will be necessary to illustrate theaction of the apparatus .

C'

oalomo’s E xperiments

- C oulomb experimented withthis apparatus to prove the law applicable to the action ofelectricity at a distance .

In one experiment his charged body deflected the discthrough an angle of H e brought the disc back half thedistance

,or to by turning the upper disc 1 2 6 ° as marked

by the index . N ow in the first case the force tending to

bring the disc back to i ts first position,wh en it was 36

°

away,was the torsion or twist of the wire

, due to the 36°

deflection of the disc. When , however, i t was brought backagainst th e same force of repulsion to 18

° by turningtheupper disc through the force then acting on the ball

,

and tending to bring it to the zero of the scale,would be 1 8 °

due to the torsion of the wire, and the force of 1 2 6°com

municated by the turning of the upper disc . T hese may becompared

,and numerically represented in the following

way

E lectronieters .

I st force due to torsion ofwire

zud force due to torsion of wire‘

2nd turning upper disc

T otal force 144°

T he intensities therefore are as

36 : I44or as

that i s at nalf t/ze distance t/ze intensity isfour times as great.

N ote— Before the T orsion Balance is used, the disc and

ball must be in contactWi th each other at the zero of thegraduated circle and the pointer of the upper disc mustbe also at its z ero.

T he Peltier E lectrometer.

T he action of the Pelt ier electrometer is based on a comparison of the repuls ion of a movable rod from the chargeof electricity communicated to the apparatus of which itforms a part, and

’

the'

position to which i t is brought by theinfluence of the earth ’s magnetism upon the small magnet

on which the rod l ies .

SirWil liam T hompson’s Quadrant E lectrometer.

T his i s a more delicate ins trument than either of thosealready described, and gives us an easy and accurate method

of determining the character and intensity of the electricalcharge ; but as it belongs properly to the advanced por

tion of the subj ect, i t will only be thus briefly referred to

here.

E xperiments— E xhibit tension of e lectricity of prime conductor

under d ifferent conditions by Henley’s Quadran t E lect rometer.T est other e lectrometers if possible .

16. A tmospheric E lectricity.

T he electricity of the atmosphere is sometimes positiveand sometimes negative. When the sky is clear, i t is always

I 2

T ex t-Boole of F rictional E lectricity .

positive but during°

the formation of rain,hail

,snow

,mist

'

and fog, i t. i s sometimes negative with frequent changes topos it ive and from pos itive again to negative , at times exceedingly rapid. It is thus possible for two masses of cloudnear one another to be in opposi te e lectrical states ; and ,when the tension becomes sufficiently high, they dischargetowards one another by means of a visible fl as/z, which wename lightning, and we hear a report orpeal termed tnunder.

Again,the earth is the great reservoir of neutral electricity ;

but the surface of the earth beneath highly charged cloudshas the opposite electricity to that of the clouds induced on

i t . If the electricity of the clouds be positive,that of the

earth beneath them will be negative ; but, if the clouds benegative , the earth ’s surface will be positive . Should thesetwo electricities . be kept from discharging gradually andsilently towards one another by a layer or a series of layers

of dry air, the tension of each increases according to thedevelopment of electrici ty in the clouds . If this be rapid

,

th e attraction of the two electricities for one ano ther soonbecomes so strong that, l ike the two e lectricities on eachside of the condenser, and L eyden jar, they sometimes piercetlze dielectric separa ting t/zein

, and produce what i s described (p . 6 1 ) as a disruptive discnarge. T his generallytakes the form of a long zigzag spark, and is due to the

interchange of electricity from the clouds to the earth,and

from the earth to the clouds . We call this spark lightning,

and it is\the most dangerous form of e lectrical discharge .

In considering atmospheric electricity we cannot fail tonotice the resemblance between the development of athunderstorm ,

and th e charging of a fulminating pane or

L eyden jar. T he clouds on which the electricity is producedresemble the action of the collecting plate , the surface of

th e earth the condens ing plate, and the dry air betweenthem the dielectric separating the metallic surface s .

T ext-Boole of Frictional E lectricity.

T hunder.

T hunder is the report which is heard when a dischargeof atmospheric electrici ty takes place . If the discharge benear

,i t follows the flash almost instantaneously. I t then

produces most frequently a violent clash,known as a thunder

clap . When the discharge takes place at a distance fromthe spectator, an interval of time elapses after the flashis seen before the thunder is heard . T his sometimesamounts to several seconds

,and arises from the fact that

sound does not travel so rapidly as light . T he velocity of

l ight over portions of the earth ’s surface within our range of

vis ion is practically instantaneous but sound, at the ordinarytemperature of the air

,travel s only feet in a second .

When, therefore, a certain number of seconds elapse between seeing the l ightning and hearing the thunder

,we know

that the discharge has taken place at a distance equal tothe distance travelled by sound per second multipl ied by thenumber of seconds . Suppose, for example, 1 0 seconds were

counted, the discharge would then have taken place

feet x 1 0, or feet away. As this i s a little more than

two miles nearly we may say roughly that for every fiveseconds counted, one mile must be allowed . When the

thunder comes from a distance i t i s heard as a rumblingsound, increasing and decreasing m loudness . T his is due

in part to the echoes or reflection of sound from the clouds,and at times to the flash producing centres of explosion ,some of which are nearer to the observer than others , andthe action of the sound-waves upon one another sometimesincreasing

,and at other times lessening

,the intensi ty of the

sound . Accidents from lightning are most common whenthe thunder follows close upon the flash .

T /ze R eturn S/zoc/a— Sometimes , however, accidents happen by what i s termed t/ie return snock

,when the discharge

has taken place at a distance from the injured person . T hisi s due to the recombination of the two electricities of thehuman

‘

body after the person has been under the inductive

T /ze R eturn S /zocé.

influence of a h ighly e lectrified cloud . A body is somet imes partially insulated

,but is in such a condition that the

repelled e lectricity may pass gradually to,and the opposite

kind be attracted as gradually from,the earth . T he per

son thus becomes slowly but highly charged with the oppos ite kind of electricity to that of th e cloud above . T he

electricity of the cloud is instantly discharged, and the personas quickly returns to the neutral condition by the oppositeelectrici ty leaving the body and passing to the earth

,while

the repelled e lectricity similar to that of the cloud re turns .T he shock produced by this return of the body to the neutralcondition is some times so vio lent

,that persons have been

kno cked down,and even killed by it but fatal accidents

from this cause are not common . T here i s another formof the return shock

,which arises from the person be ing

perfectly insulated and under the inductive action of a high ly

electrified cloud . If the electricity of the cloud be positive,the upper port ion of the body becomes negative , and the

lower pos it ive . When the cloud is discharged, these two

electricities,which were only decomposed, recombine sud

denly, and at times so violently that a sensible shock is felt .

Although the discharge of atmospheric electricity is

sometimes dangerous to human beings, and destructive to

buildings,protection may be afforded by the use of suitable

rods,properly arranged

,and attached to portions of th e

buildings which we desire to render secure . Such rods arenamed L ightning C onductors .

L ightning C onductors .

T he l ightning conductor consists of three parts

1 . T he rod .

2 . T he conductor .

3. T he earth contact, orportion in the ground .

1 . T /ze rod i s the part above the building, which shouldterminate in a very fine point . A thin platinum wire 1s

T ext-Boon of F rictional E lectricity.

generally used for the top of i t. T he finer the po int themore perfect i s the action of th e arrangement. Should thepo int of the rod be iron

,it i s gilded to prevent it from rusting.

2 . T /ie conductor i s usually made of copper or iron .

C opper is the best conducting substance, but iron improvesafter a time

,whereas copper becomes more brittle . T he

best material for a conductor is a galvanised iron wire .

Straps of copper are also used with good results . If aniron cylindrical conductor be used, {

37 in . in diameter will be

sufficient but if copper, gm.

3. T ne cart/i contact — T his i s a very important part ofth e l ightning conductor

,and unless i t be carefully attended

to, and the conductor led away into mo ist earth, the erectingof such anarrangement against a building will be a sourceof danger

,rather than protection . T he conductor should

be taken for a little distance into the earth, and then turnedat right angles to the building

,and connected with the gas

or water-pipes,or taken into a well or moi st earth . If this

canno t be secured,a narrow and deep pit should be dug,

and the conductor led into i t, and packed around withfresh charcoal . C harcoal should also surround i t, as itextends from the building to the pi t in which i t terminates .

Use of Conductors .

T he conductor acts on a highly electrified cloud in asimilar way to a po int

,he ld in the hand

,on th e charged

prime conductor of an electrical machine . I t enables one

kind of e lectricity to pass from the cloud to the earth,and

th e oppos ite kind to pass from the earth to the cloud thusreducing the tens ion

”

of the electric ity of the cloud,and

robbing i t of i ts tendency to be dangerous in two ways .T he distance rendered secure by a well-constructed lightningconductor is the area included within a circle, whose radiusi s the length of the height of the conductor from the ground .

L arge buildings should have more than one conductor

attached to them to afford perfect security .

T ex t-Boon of Frictiona l E lectricity.

If possible a catskin should be got, a fo x’s brush

,and

some gun cotton .

T he followmg will be found useful —a book of Dutchmetal, a pennyworth of thin copper wire, a long lath, somestring, hoops of cartridge paper, and an empty egg-shell .A little shellac varnish should also be obtained for

painting the glass rods, used as supports for insulating con

ductors.

T his may be easily and cheaply made .

T o make Shel lac Varnish .

Pour the same weight of methylated spirit on shellac, andlet i t remain for 24hours . T hen dilute it with two or th reetimes as much spiri t . Strain it through flannel and keep itbottled . T o varnish articles

,heat them

,so that they may

be sensibly warm,but not too hot draw the brush regu

larly in the same direction don’t disturb the varnish, butlet i t dry

,or i t will not l ie smoothly .

T o make an E lectric Pendulum.

F ix glass rod coated with shellac varnish in wooden base .

F it brass cap to rod,and attach brass wire bent as in

d iagram,fig . 9, and terminating in a hook . F asten pith

ball to a fibre of silk, and suspend it from the hook . T he

stand may be made by fastening a stick of sealing-wax to atin plate . T his can be done best by heating both the plateand the sealing-wax . A piece of bent wire may then bestuck into the end of the sealing-wax .

T o make aGold-L eaf E lectroscope.

Get a glass jar, or large bo iling flask . F it a cork into i t,

and pass through the cork,insulated from it by a coating of

gutta-percha or other good insulator,a brass rod . Solder a

brass or copper disc,having a small hole near its edge, to

the end of the rod . Attach also a strip of metal,say brass

,

at the other end of the rod,and at right angles to i t . Stick

S ugges tions for Appara tus . 123

with gum or paste two equal strips of gold leaf or Dutchmetal . T hey should be about gin . wide

,and 2 in . long .

T o make a Proof Plane .

C ut out two pieces of thin cardboard, one circular

,

diameter in .,and the o ther oblong 3 in . by 2 in . ;

round off corners of latter,cover each smoothly on both

sides with tin-fo il, and fasten separately to a strip of glassabout 9 in . long and 5 in . wide ; coat glass strips withshellac varnish . P ieces of gilt or s i lvered paper may be usedinstead

,or discs of thin metal .

T o make an E lectrophorus.

Obtain circular base of ebonite,gutta-percha

,or resin

,or

even coarse brown paper ; let i t have a diameter of about

9 in. T o the centre of a disc of tin or thin brass , 7 in . in

diameter,fasten a sealing-wax , gutta-percha, orglass handle ,

about 6 in . long. Should the handle be glass, a coating ofshellac varnish will be necessary.

Conductors.

T he various shaped conductors may be made of wood,and coated with tin-fo il . T his must be done smoothly, so

as to avo id edges or po ints,o therwise the e lectricity will

rapidly escape . T hey may be supported on glass rodscoated with shellac varnish .

Such bodies as apples,turnips

,potatoes , and carrots, it

smooth and warmed,may be used as conductors, and can

be supported by a needle projecting from a stick of sealing

wax. T he sealing-wax should be fastened to a tin disc to

support i t .

T o form an I nsulating Stool .

An ordinary drawing-board well warmed, and placed on

four glass tumblers, will serve for this . I t i s advisable tocoat the tumblers wi th shellac varnish to secure more perfectinsulation

,if not coated they should be well warmed .

T ex t-Boon of Frictiona l E lectricity .

T o make a F araday’

s Butterfly N et.

F orm a circle with fairly thick wire the diameter of thecircle should be about 4in . L et the two ends of the w ire ,after the circle i s completed

,be secured with shellac in a

piece of glass tubing, 9 in . long and i in . in diameter .

Make a conical gauze bag, with base 4 in . and length

9 in .

,and attach it to the circle . F asten two s ilk strings ,

each about 1 8 in . long, one to the inside and the other to

the outside of the apex of the cone.

T o make a F ulminating Pane, or F rankl in’s Plate.

C oat both sides of a sheet of glass 1 2 in . by 9 in. ,

with equal-s ized sheets of tin-fo il, so that they may cover oneanother e xactly. T he fo i l should be 8 in . by 5 ih .

,so

as to allow a 2 in . border of glass all round the fo il . C oat

th e glass border with shellac varnish .

T o make a L eyden J ar.

Paste tin-fo il inside and outside of a wide-mouthed, thinglass bottle

,to about 2 in . from the top. F it large cork or

wooden stopper into the mouth of the j ar . Pass thick brasswire through centre of stopper . Attach a knob to one end of

wire,and a piece of chain or wire to the other . T he chain

or wire must be in contact with the inner coating of the j ar.

T o make a Discharging R od.

F IG. 95 ,

Get a good-sized test tube . Bend a pieceof copper wire 5 in . thick into the shape re

presented in diagram,fig. 95 . Solder two

leaden bullets to the ends . Secure bent por

tion in the test tube with shellac.

Discharging rod .

T o make a L eyden J ar with movable coatings .

Obtain a smoo th glass tumbler, have two t in cups fitted

to the outside and inside . A thick in . ) brass wire should

T ex t-Boon of Frictiona l E lectricity.

C onduction and insulation — T hese were the results of

the i nvestigations of Stephen Gray,A .D . 1 7 2 9. H e also

noticed the e lectric brush,snappings

,and sparks

,and ex

perimented on induction .

Vitreous and P esinous E lectricity— T hese two kinds of

electricity were first announced by Du F ay, about A .D . 1 733,

and his name became attached to what was afterwardsknown as tne twofluid t/zeory.

P ositive and M gatioe.

— F ranklin,A .D . 1 747 , substituted

the terms ‘ pos itive ’ and ‘ negative for vitreous and‘ res inous in connection with his onefl uid tneory.

Symmer’

s T neory.

— Symmer adapted Du F ay’s theory to

th e positive and negative idea introduced by F ranklin ; andhenceforth the two fluid theory became more immediatelyconnected with the name of Symmer.

T ne L eydenf an— T hisjar has received its name from itsbeing discovered in a rough form by C unaeus

,a rich burgess

of L eyden,A .D . 1 746 . T yndall remarks

,

‘ I t was first announced in a letter addressed on the 4th of N ovember

,1 745 ,

to Dr. L ierberkiihn, of Berlin,by"leis t

,a clergyman of

C ammin in Pomeran ia .

’ Its present form is due to the investigations of Dr. Bevis, and D r. Watson, but chiefly thelatter .

T ne Unit [an— T he two chief bear the name of Harrisand L ane .

E lectrical L aw s — T hese were established by C oulombby means of experiments with the T orsion Balance, A .D .

1 78 7 .

Tne E lectropnorus and C ondensing E lectroscope.

— Boththese apparatus bear the name of Volta. T he former wasinvented A .D. 1 7 7 5 , and the later A .D . 1 78 2 .

T lie E lectroscope.

— T he Gold-L eaf E lectroscope i s due toBennett

,and is sometimes named Bennett’s electroscope.

A tmospheric E lectricity. F ranklin was the first andgreatest investigator in atmospheric electricity . H e wasfollowed by D’

A libard , and R ichmann, of St . Petersburg.

‘

Brief H is tory of Frictiona l E lectricity. I 27

T he latter met his death from lightning, while experimentingwith an insulated rod

,A .D . 1 7 53.

Z zgntnzng C onductors — T hese were first introduced byF ranklin, A .D . 1 749, and after much discuss ion were adopted .

A lightning conductor was first attached to St . Paul’s C athedral . Another prominent name connected with lightningconductors , particularly those of ships

,i s that of Sir Snow

Harris,of Plymouth. Since then various eminent men have

investigated different portions of this important science,and

have aided its development by thei r studies and experiments .F oremost amongst a long array stand the names of SirHumphry Davy

,F araday

,T yndall

,Guthrie

,and Sir

William T hompson .

UN I VE R S I T 1

SC IE N C E A N D A R T DE PAR T ME N T QU E ST ION S .

F R IC T ION AL E L E C T R IC IT Y .

ELEMENT ARY ST AGE .

186 7 .

1 . We often hear of an elec tric fluid .

’ What are your no tions regarding i t ? P . 9.

2 . I f asked to show some of the e lementary effects of e lectrici ty, howwould you do i t ? P . 3.

How is e lectrici ty stored in the Leyden j ar ? Pp . 68 —73We hear of frictional e lectricity , and of vo l taic e lectrici ty . Whatis th e difference be tween them ? Describe the ways in whichboth of them may be obtained . Partly p . 3.

What are l ightning-conductors ? and how do they act in avertingdanger from lightning ? Pp . 1 19—120 .

1 868 .

I . What is meant by the term insulated ? I bring a glass rod whichhas been rubbed w i th silk near an insulated brass sphere whatis th e condition of th e sphere while the glass is near it ? Whatoccurs when th e glass is removed ? Pp . 32 33.

I f you take an insulator in your hand and rub i t properly you e lectrify i t if you do the same with a conducto r you obtain no signsofe lectrici ty what is th e reason Bodie s were formerly dividedinto electrics and non-e lectrics why ? Pp . 26—2 7 .

E xplain in accordance with the theory of e lectric fluids the attractionof light bodies by an e lectrified glass rod . Pp . 36—37 .

Describe and explain the original experiment which led to th e

d iscovery of the Leyden jar . Pp . 66—6 7 .

Describe some one form of e lectric machine,and explain its use .

Pp . 46-47 .

1 869.

I . An e lectrified body is brought near a scrap of paper or of gold leafre sting on a metallic surface ; what happens to the scrap , andwhat is its exact electrical condition when it is attracted Sup

T ex t B oon of Frictional E lectricity.

1 8 7 2 .

1 . I f upon a warm board a dry shee t of paper be rubbed with india~rubber i t i s e lectrified How is this proved ? P . 3.

2 . Show by a simple experiment that th e e lectricity deve loped on resinby the fric tion of flannel is different from that developed on glassby the friction of silk . Pp . 12 .- I4.

Describe Franklin ’s Plate,and explain i ts action . Pp . 64—65 .

A stripof paper r ubbed with indiarubber i s brought near to a glas srod which has been rubbed with silk : what fo llows ? Deducefrom the experiment the quali ty of the e lectrici ty upon the paper.P . 14.

E xplain fully what takes place when light bodies are attrac ted by a

stick of sealing-wax rubbed with flannel . Pp . 4, 37 .

1 8 73.

1 . I hold a dry glass rod , which has been rubbed with silk , near a

brass ball , which is supported on a d ry glass stand what is thestate of the ball ? Supposing th e stand which holds the brassbal l to be mo ist instead of dry what will occur ? Pp . 32

—33.

2 . T wo st rings are given to you , and you are required to test whe therthey insulate or conduct e lectricity ; how wil l you do i t ? Pp .

T w o brass balls supported on glass stands are united by a chain ;a stick of sealing-wax rubbed by flanne l i s brought near one of

the bal ls what is th e condition of th e o ther ? Pp . 40 —41 .

Y ou touch th e bal l furthest from th e sealing-wax , remove yourhand

,and then remove th e sealing-wax ; what is now th e e lec

trical condition of th e tw o balls and the chain ? Y ou must statewhat occurs at each step of the process . P . 33.

Y ou are required to charge and discharge a Leyden j ar ; how willyou do i t ? Pp . 67—7 2 .

T here are tw o kinds of e lectrici ty, called posit ive and negative .

Y ou rub sealing-w ax with flannel are bo th e lectrici tie s excited ,or are they not ? I f bo th , where ? I f only one

,where ? P . 2 2 .

1 874.

I . A collodion bal loon simply stroked with the hand becomes negatively e lectrified . Supposing you were asked to prove the

t ruth of this statement , how would you proceed ? P . 14.

2 . An egg-she l l i s placed on a table , and a glass rod which has beenrubbed with silk is brought near th e she l l ; th e she l l rolls afterth e rod . Describe th e condition of th e rod and the shel l duringthe mo tion of th e lat ter .

3. Y ou a re required to prove by experiment the e lectrical law that

Ques tio ns zn Frictiona l E lectricity. 13I

bodies similarly e lectrified repe l ; and that bodies dissimilarlye lectrified attrac t each o ther -h ow will you do i t Pp . 23-24.

Describe th e Leyden j ar and themode of charging i t . Pp . 6 7-69.

5 . Describe the e lectrophorus and th e mode of charging it . Pp . 42-44.

18 75 .

I . From what is the word e lectrici ty derived ? Describe the substancewith which the first e lectrical effects were obse rved

,and describe

also the mode of exciting it . P . 1 .

2 . Y ou are required to e lectrify strongly a glass tube how will you doi t Y ou are required to electrify strongly a tube of gut ta-perchahow will you do i t ? Pp . 4, 14.

What is the action of tw o e lectrified glass tubes upon each o ther ?What is th e action of tw o e lectrified gutta-percha tubes uponeach o the r ? -V Vhat is the action of an e lectrified glass tube uponan e lectrified tube of gutta-percha ? P . 4.

I stick a bi t of sealing-wax against a penny or a halfcrown and Iwhisk a bit ofvulcanised indiarubber with a fox’s brush . Holdingth e sealing-wax as a handle

,I lay th e penny or halfcrown flat

on the indiarubber. What is the condition of the coin ? I touchth e co in , what occurs I lift i t by the handle

,what is its condi

tion ? Pp . 42 -44.

Y ou are required to explain , in accordance with th e theory of

e lectric fluids , how l ight bodies are attracted by a glass rod positively e lectrified , and by a gutta-percha tube negatively electrified ?

18 76.

1 . \Vhat i s meant by the terms conduction and insu lation as appliedto frictional e lectricity ? Describe an experiment which shal lil lustrate th e properties of me tal wire , common twine , and asilk string as regards conduction and insulation . Pp . 2 5—30 .

2 . A shee t of hot paper is placed on a board , and indiarubber i s passedbriskly over i t . T w o strips are cut from the paper and held upclose and paralle l to each other . How will they act upon eacho ther ? A glass rod rubbed with silk repels bo th the stripswhat is th e inference ? -Pp . 7- 8 .

Describe and explain th e action of th e common gold-leaf e lectroscope .

PAn fpple he ld in the hand and struck with a fox ’s brush shows nosigns of e lectrical action suspended by a silk string and struckwith the brush i t becomes electrified , attracting light bodies , andcausing th e leaves of th e e lectroscope to diverge . E xplain theseresults . Pp . T /ze apple is a condu ctor li/ee [grass ro

d .

A stout stick of sealing-wax i s stuck upright to a p iece of woodacting as a bas e into the wax at th e top is inserted a needle , andon to th e needle is fixed an apple . Near to the apple , but no tinto contac t with it , is brought a rod of glass Wt ll has beenrubbed by silk .

What is th e condition of the apple while the

T ex t-Boonof Frictiona l E lectricity.

rod remains near i t ? What occurs when the apple is touchedfor a moment ? What finally occurs when the rubbed glass isw ithdrawn ? P. 36 .

1 87 7.

I . A lath , six fee t long, is supported at i ts centre on a dry glass tumbler . Below one end of the lath , and at a distance of someinches from it , are placed some scraps of gold leaf or o ther l ightbod ies . A glass rod e lectrified by friction is brought over th eo ther end of the lath without touching i t T he fragments of

go ld leaf are immediately at tracted . H ow I S this at traction produced

2 . Y ou are required to give an experimental proof of the law thatbodies oppositely e lectrified attract each o ther , and bodies similarly e lectrified repe l each o ther . Te l l me the substances youwould choose , and th e manner in which you °

would use them to

obtain th e required resul t . Pp . 12—14.

A thunder-cloud charged positively comes over a po inted lightningconductor. T h e cloud generally loses i ts charge of e lectricity byth e action of th e conductor . H ow i s this accomplished P . 120 .

Y ou are required to make a small Leyden j ar : how will you pro

oecd ? Y ou are required to charge th e j ar when made : howwill you do i t ? Pp . 67—70 .

Y ou are required to explain fully what occurs during the chargingand discharging of your Leyden j ar. Pp . 67—72 .

1 8 7 8 .

1 . G ive a clear definition of the'

terms e lectric condu ction and e lectricindu ction . Pp . 28 , 29, 34.

Supposing you were required to develop induced e lectricity and toprove i ts ex istence , how would you do i t ? P . 40 .

Supposing you were required to test the quali ty of th e e lectricitywith which an insulator is charged , how would you do i t ? P . 18 .

What occurs when you whisk th e resinous plate of an e lectrophoruswith a fox’s brush ? T he plate be ing excited , how would youobtain th e spark of the e lectropho rus ? P . 43.

Describe an experiment which shall il lustrate th e action of th e e lectrical condenser. Pp . 60—6 1 .

1 8 79.

1 . I f you rub toge ther a stick of sealing-wax and a piece of flannel , andthen put them bo th on an e lectroscope , th e leaves do not move .

What happens to the electroscope if you remove ( I ) the flannel ,( 2 ) th e sealing-wax ? What would be the effec t in each case ofbringing near th e e lectroscope a glass rod that had been rubbedwith si lk ? P. 2 2 .

2 . How would you prove that a brass tube i s negatively electrifiedwhen struck with flannel ? Pp . 8 or 18 .

T ex t-Boon of Frictiona l E lectricity.

1 88 2 .

1 . A stick of sealing-wax is rubbed with dry flannel , and held overpith-ball lying on a table . T he ball rises to th e sealing-wax athen falls again . Why does i t rise

,and why does i t fall ? Pp . 36

2 . When a piece of sealing-wax and a piece of dry flanne l are rubt

toge ther , one becomes posi t ive ly e lectrified and the o ther negtively e lectrified . When a piece of dry paper and a piece :indiarubberare rubbed toge ther one becomes positively electriftand the o ther negative ly e lectrified . How could you findwhich of the four things

,sealing-wax , flanne l , paper, indiarubb

are in the same e lectrical state ? P . 2 1 .

3. Describe a Leyden j ar and the me thod of charging i t . Pp . 67—6 1'

4. A piece of dry brown paper , laid on a warm me tal tray , is rubb

with catskin . T he t ray is then placed on a dry glass tumble—s ethe brown paper is removed . E xplain how i t is that you can 11.get a spark on bringing your knuckle near the tray. Pp . 42

Arrange the fol lowing substances in th e o rder of the ir conductlpowers for e lectricity

,putting the name of the best conduct

first : air, copper, glass , i ron , sea-water , shel lac , wate r (puiwood . Pp . 24—2 5 .

1883.

A piece of brass tube he ld in th e hand and struck with catskinno electricity when it is made to touch an e lectroscope .

would you prove experimentally that i t was really electwhen so s truck ? Pp . 26—2 7 .

2 . How is i t that in damp weather an ordinary platework wel l ? Pp . 4, 49.

In th e common plate or cylinder electrical mof rounded shape at all parts except where i t comes ne

th e plate or cylinder, but here it is provided with sharp prpoints . What reason is there for this arrangement Pp .

A little pith-ball rests on a brass plate provided with a glassT he tw o are placed on a cake of resin , which has beenwith a catskin . When th e plate i sthen lifted by the handle , the pith-balPp . 43—44.

A deep me tal pot, posmvely e lectrified , stands on a glass s tem .

me tal ball , hung by a silk thread , is pu t in contact with aleaf e lectroscope after be ing made to touch(a ) fi rst th e inside, and then the ou tside of the pot or

(b) first th e ou tside, then the ins ide of th e pot.

S tate and explain the effect on th e e lectroscope in each case .

90 - 91 .

1 884.

1 . I f you have a positively charged brass plate and a piece of gi l t pfastened to th e end of a dry glass rod , how could you chargegil t paper with negative e lectrici ty P . 36 .

Ques tions in F rictiona l E lectricity. I 35

he prime conductor of an e lectrical machine has a long brass rodprojecting from it ; from th e end of a rod a pith bal l hangs by adamp c0tton thread , and a pin is . driven into th e pith ball up toi ts head“

, so that the point projects on the o the r side . How andwhy does the ballmove when th e machine is turned ?hree insulated me tal balls , A ,

B,and C ,

are placed in a line , A and Bin contact , C a li t tle way off. C is positively e lectrified

,and then

A and B are separated . What are now the e lectrical states of Aand B ? P . 41 .

on have an electrical machine standing on a table with a glasstop, and you have no means of connecting i t e lectrically with th eearth . What would you do in o rder to charge a Leyden jar bythe machine ? P . 53.

‘nder what circumstances can you get a charge on a metal ballhanging by a silk thread by touching therewith the inside of a

me tal j ar ?1 885 .

w o pith-balls suspended , one by a damp co tton thread , th e o therby a dry silk thread , are each of them touched by th e knob of acharged Leyden j ar

,which is held in th e hand by i ts outer coat

ing. Will the re be any difference between th e behaviour of th eballs ? I f so

,what difference and why ?

th -ball s hang side by side by tw o damp co tton threads . S tateexplain what happens when an excited glass rod is broughtually near th e tw o balls from below .

to get a succession of sparksfrom an e lectrophorus . P . 44.

brass rod is supported horizontal ly by a d ry glass s tem ,and a

large , strongly e lectrified me tal ball is brought near one end of

the rod (but not near enough for a spark to pass ) . T h e rod isthen touched for an instant by th e end of an earth-connected wire ,and afterwards the ball is removed . Will i t make any differencein th e final e lectrical s tate of th e brass rod , whe ther the wiretouches i t at the end neares t th e ball , at the end farthest from the

ball , or at the middle ? G ive reasons for your answer. Pp . 33,

34, 36ng the knob of a charged Leyden jar s tanding on the floor ,

lectric shock ; but if e i theresin , you do notget a shock

t/ze conducting connection betw een t/ze ou ter and inner

coatings , and t/zu s prevent t/ze interchange of til e tw o electru itzes ,

and til e disc/l arge of tnejar.

PR I N TED B Y

s porrrswoonn A N D co . , N EW-STR E ET SQUA R EL ONDON