-
History of electromagnetic theory
For a chronological guide to this subject, seeTimeline of
electromagnetic theory.
The history of electromagnetic theory begins with an-cient
measures to deal with atmospheric electricity, inparticular
lightning.*[1] People then had little under-standing of
electricity, and were unable to scienticallyexplain the
phenomena.*[2] In the 19th century therewas a unication of the
history of electric theory withthe history of magnetic theory. It
became clear thatelectricity should be treated jointly with
magnetism, be-cause wherever electricity is in motion, magnetism is
alsopresent.*[3] Magnetism was not fully explained until theidea of
magnetic induction was developed.*[4] Electric-ity was not fully
explained until the idea of electric chargewas developed.
1 Ancient and classical historyThe knowledge of static
electricity dates back to the ear-liest civilizations, but for
millennia it remained merelyan interesting and mystifying
phenomenon, without atheory to explain its behavior and often
confused withmagnetism. The ancients were acquainted with
rathercurious properties possessed by two minerals, amber(Greek: ,
electron) and magnetic iron ore(Greek: , Magnes lithos, the
Magne-sian stone, lodestone). Amber, when rubbed, attractslight
bodies; magnetic iron ore has the power of attractingiron.*[5]Based
on his nd of an Olmec hematite artifact in CentralAmerica, the
American astronomer John Carlson hassuggested thatthe Olmec may
have discovered and usedthe geomagnetic lodestone compass earlier
than 1000BC. If true, thispredates the Chinese discovery ofthe
geomagnetic lodestone compass by more than a mil-lennium.*[6]*[7]
Carlson speculates that the Olmecsmay have used similar artifacts
as a directional device forastrological or geomantic purposes, or
to orient their tem-ples, the dwellings of the living or the
interments of thedead. The earliest Chinese literature reference to
mag-netism lies in a 4th-century BC book called Book of theDevil
Valley Master (): The lodestone makesiron come or it attracts
it.*[8]Long before any knowledge of electromagnetism existed,people
were aware of the eects of electricity. Lightningand other
manifestations of electricity such as St. Elmo'sre were known in
ancient times, but it was not under-
The discovery of the property of magnets.Magnets were rst found
in a natural state; certain iron ox-ides were discovered in various
parts of the world, notably inMagnesia in AsiaMinor, that had the
property of attracting smallpieces of iron, which is shown
here.
Electric catsh are found in tropical Africa and the Nile
River.
stood that these phenomena had a common origin.*[9]Ancient
Egyptians were aware of shocks when interactingwith electric sh
(such as the electric catsh) or other ani-mals (such as electric
eels).*[10] The shocks from animalswere apparent to observers since
pre-history by a varietyof peoples that came into contact with
them. Texts from2750 BC by the ancient Egyptians referred to these
sh
1
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2 2 MIDDLE AGES AND THE RENAISSANCE
asthunderer of the Nile" and saw them as theprotec-torsof all
the other sh.*[5] Another possible approachto the discovery of the
identity of lightning and electric-ity from any other source, is to
be attributed to the Arabs,who before the 15th century used the
same Arabic wordfor lightning (barq) and the electric
ray.*[9]Thales of Miletus, writing at around 600 BC, noted
thatrubbing fur on various substances such as amber wouldcause them
to attract specks of dust and other light ob-jects.*[11] Thales
wrote on the eect now known as staticelectricity. The Greeks noted
that if they rubbed the am-ber for long enough they could even get
an electric sparkto jump.The electrostatic phenomena was again
reported mil-lennia later by Roman and Arabic naturalists
andphysicians.*[12] Several ancient writers, such as Pliny theElder
and Scribonius Largus, attested to the numbing ef-fect of electric
shocks delivered by catsh and torpedorays. Pliny in his books
writes: The ancient Tuscansby their learning hold that there are
nine gods that sendforth lightning and those of eleven sorts.This
was ingeneral the early pagan idea of lightning.*[9] The
ancientsheld some concept that shocks could travel along
conduct-ing objects.*[13] Patients suering from ailments such
asgout or headache were directed to touch electric sh in thehope
that the powerful jolt might cure them.*[14]A number of objects
found in Iraq in 1938 dated to theearly centuries AD (Sassanid
Mesopotamia), called theBaghdad Battery, resembles a galvanic cell
and is believedby some to have been used for electroplating.*[15]
Theclaims are controversial because of supporting evidenceand
theories for the uses of the artifacts,*[16]*[17] physi-cal
evidence on the objects conducive for electrical func-tions,*[18]
and if they were electrical in nature. As a re-sult the nature of
these objects is based on speculation,and the function of these
artifacts remains in doubt.*[19]
2 MiddleAges and theRenaissanceMagnetic attraction was once
accounted by Aristotle andThales for as the working of a soul in
the stone.*[20]In the 11th century, the Chinese scientist Shen
Kuo(10311095) was the rst person to write of the mag-netic needle
compass and that it improved the accuracyof navigation by employing
the astronomical concept oftrue north (Dream Pool Essays, AD 1088
), and by the12th century the Chinese were known to use the
lodestonecompass for navigation. In 1187, Alexander Neckam wasthe
rst in Europe to describe the compass and its use
fornavigation.Magnetism was one of the few sciences which
progressedin medieval Europe; for in the thirteenth century
PeterPeregrinus, a native of Maricourt in Picardy, made adiscovery
of fundamental importance.*[21] The French13th century scholar
conducted experiments on mag-
Shen Kua wrote Dream Pool Essays (); Shen also rstdescribed the
magnetic needle.
netism and wrote the rst extant treatise describing
theproperties of magnets and pivoting compass needles.*[5]The dry
compass was invented around 1300 by Italian in-ventor Flavio
Gioja.*[22]Archbishop Eustathius of Thessalonica, Greek scholarand
writer of the 12th century, records thatWoliver, kingof the Goths,
was able to draw sparks from his body.The same writer states that a
certain philosopher was ablewhile dressing to draw sparks from his
clothes, a resultseemingly akin to that obtained by Robert Symmer
in hissilk stocking experiments, a careful account of whichmaybe
found in the 'Philosophical Transactions,' 1759.*[9]Italian
physician Gerolamo Cardano wrote about electric-ity in De
Subtilitate (1550) distinguishing, perhaps for therst time, between
electrical and magnetic forces.Toward the late 16th century, a
physician of Queen Eliz-abeth's time, Dr. William Gilbert, in De
Magnete, ex-panded on Cardano's work and invented the New Latinword
electricus from (elektron), the Greekword foramber. Gilbert, a
native of Colchester, Fel-low of St John's College, Cambridge, and
sometime Pres-ident of the College of Physicians, was one of the
earliestand most distinguished English men of sciencea manwhose
work Galileo thought enviably great. He was ap-pointed Court
physician, and a pension was settled on himto set him free to
continue his researches in Physics andChemistry.*[23]Gilbert
undertook a number of careful electrical exper-iments, in the
course of which he discovered that manysubstances other than amber,
such as sulphur, wax, glass,etc.,*[24] were capable of manifesting
electrical proper-ties. Gilbert also discovered that a heated body
lost itselectricity and that moisture prevented the electricationof
all bodies, due to the now well-known fact that mois-ture impaired
the insulation of such bodies. He also no-ticed that electried
substances attracted all other sub-
-
3stances indiscriminately, whereas a magnet only attractediron.
The many discoveries of this nature earned forGilbert the title of
founder of the electrical science.*[9]By investigating the forces
on a light metallic needle, bal-anced on a point, he extended the
list of electric bodies,and found also that many substances,
including metalsand natural magnets, showed no attractive forces
whenrubbed. He noticed that dry weather with north or eastwind was
the most favourable atmospheric condition forexhibiting electric
phenomenaan observation liable tomisconception until the dierence
between conductorand insulator was understood.*[23]
Robert Boyle.
Gilbert's work was followed up by Robert Boyle (16271691), the
famous natural philosopher who was oncedescribed asfather of
Chemistry, and uncle of the Earlof Cork.Boyle was one of the
founders of the RoyalSociety when it met privately in Oxford, and
became amember of the Council after the Society was incorpo-rated
by Charles II. in 1663. He worked frequently at thenew science of
electricity, and added several substancesto Gilbert's list of
electrics. He left a detailed account ofhis researches under the
title of Experiments on the Ori-gin of Electricity.*[23] Boyle, in
1675, stated that electricattraction and repulsion can act across a
vacuum. Oneof his important discoveries was that electried bodies
ina vacuum would attract light substances, this indicatingthat the
electrical eect did not depend upon the air as amedium. He also
added resin to the then known list
ofelectrics.*[9]*[25]*[26]*[27]This was followed in 1660 by Otto
von Guericke, who in-vented an early electrostatic generator. By
the end of the17th Century, researchers had developed practical
means
of generating electricity by friction with an
electrostaticgenerator, but the development of electrostatic
machinesdid not begin in earnest until the 18th century, when
theybecame fundamental instruments in the studies about thenew
science of electricity.The rst usage of the word electricity is
ascribed to SirThomas Browne in his 1646 work, Pseudodoxia
Epidem-ica.The rst appearance of the term electromagnetism onthe
other hand comes from an earlier date: 1641.Magnes,*[28] by the
Jesuit luminary Athanasius Kircher,carries on page 640 the
provocative chapter-heading:"Elektro-magnetismos i.e. On the
Magnetism of am-ber, or electrical attractions and their causes(-
id est sive De Magnetismo electri, seu elec-tricis attractionibus
earumque causis).
3 18th century
3.1 Improving the electric machine
Main article: electrostatic machineThe electric machine was
subsequently improved by
Generator built by Francis Hauksbee.*[29]
Francis Hauksbee, Litzendorf, and by Prof. Georg
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4 3 18TH CENTURY
Matthias Bose, about 1750. Litzendorf, researching forChristian
August Hausen, substituted a glass ball for thesulphur ball of
Guericke. Bose was the rst to employ theprime conductorin such
machines, this consisting ofan iron rod held in the hand of a
person whose body wasinsulated by standing on a block of resin.
Ingenhousz,during 1746, invented electric machines made of
plateglass.*[30] Experiments with the electric machine werelargely
aided by the discovery of the property of a glassplate, when coated
on both sides with tinfoil, of accu-mulating a charge of
electricity when connected with asource of electromotive force. The
electric machine wassoon further improved by Andrew Gordon, a
Scotsman,Professor at Erfurt, who substituted a glass cylinder
inplace of a glass globe; and by Giessing of Leipzig whoadded a
rubberconsisting of a cushion of woollenmaterial. The collector,
consisting of a series of metalpoints, was added to the machine by
Benjamin Wilsonabout 1746, and in 1762, John Canton of England
(alsothe inventor of the rst pith-ball electroscope) improvedthe
eciency of electric machines by sprinkling an amal-gam of tin over
the surface of the rubber.*[9]
3.2 Electrics and non-electrics
In 1729, Stephen Gray conducted a series of experi-ments that
demonstrated the dierence between conduc-tors and non-conductors
(insulators), showing amongstother things that a metal wire and
even pack thread con-ducted electricity, whereas silk did not. In
one of his ex-periments he sent an electric current through 800
feet ofhempen thread which was suspended at intervals by loopsof
silk thread. When he tried to conduct the same ex-periment
substituting the silk for nely spun brass wire,he found that the
electric current was no longer carriedthroughout the hemp cord, but
instead seemed to vanishinto the brass wire. From this experiment
he classiedsubstances into two categories: electricslike
glass,resin and silk andnon-electricslike metal and
water.Electricsconducted charges whilenon-electricsheldthe
charge.*[9]*[31]
3.3 Vitreous and resinous
Intrigued by Gray's results, in 1732, C. F. du Fay beganto
conduct several experiments. In his rst experiment,Du Fay concluded
that all objects except metals, animals,and liquids could be
electried by rubbing and thatmetals,animals and liquids could be
electried by means of anelectric machine, thus discrediting
Gray'selectricsandnon-electricsclassication of substances.In 1737
Du Fay and Hauksbee independently discoveredwhat they believed to
be two kinds of frictional electricity;one generated from rubbing
glass, the other from rubbingresin. From this, Du Fay theorized
that electricity consistsof two electrical
uids,vitreousandresinous, that
are separated by friction and that neutralize each otherwhen
combined.*[32] This two-uid theory would latergive rise to the
concept of positive and negative electricalcharges devised by
Benjamin Franklin.*[9]
3.4 Leyden jar
Pieter van Musschenbroek.
The Leyden jar, a type of capacitor for electrical en-ergy in
large quantities, was invented independently byEwaldGeorg vonKleist
on 11October 1744 and by Pietervan Musschenbroek in 17451746 at
Leiden Univer-sity (the latter location giving the device its
name).*[33]William Watson, when experimenting with the Leydenjar,
discovered in 1747 that a discharge of static elec-tricity was
equivalent to an electric current. Capacitancewas rst observed by
Von Kleist of Leyden in 1754.*[34]Von Kleist happened to hold, near
his electric machine,a small bottle, in the neck of which there was
an ironnail. Touching the iron nail accidentally with his otherhand
he received a severe electric shock. In much thesame way
Musschenbroeck assisted by Cunaens receiveda more severe shock from
a somewhat similar glass bot-tle. Sir WilliamWatson of England
greatly improved thisdevice, by covering the bottle, or jar,
outside and in withtinfoil. This piece of electrical apparatus will
be easilyrecognized as the well-known Leyden jar, so called bythe
Abbot Nollet of Paris, after the place of its discov-ery.*[9]In
1741, John Ellicottproposed to measure the strengthof electrication
by its power to raise a weight in one scaleof a balance while the
other was held over the electri-ed body and pulled to it by its
attractive power. TheSirWilliamWatson alreadymentioned conducted
numer-
-
3.5 Late 18th century 5
ous experiments, about 1749, to ascertain the velocity
ofelectricity in a wire. These experiments, although per-haps not
so intended, also demonstrated the possibility oftransmitting
signals to a distance by electricity. In theseexperiments, the
signal appeared to travel the 12,276-footlength of the insulated
wire instantaneously. Le Monnierin France had previously made
somewhat similar exper-iments, sending shocks through an iron wire
1,319 feetlong.*[9]About 1750, rst experiments in
electrotherapeutics weremade. Various experimenters made tests to
ascertainthe physiological and therapeutical eects of
electricity.Demainbray in Edinburgh examined the eects of
elec-tricity upon plants and concluded that the growth of twomyrtle
trees was quickened by electrication. These myr-tles were electried
during the whole month of Oc-tober, 1746, and they put forth
branches and blossomssooner than other shrubs of the same kind not
electri-ed..*[35] Abb Mnon in France tried the eects ofa continued
application of electricity upon men and birdsand found that the
subjects experimented on lost weight,thus apparently showing that
electricity quickened theexcretions.*[36]*[37] The ecacy of
electric shocks incases of paralysis was tested in the county
hospital atShrewsbury, England, with rather poor success.*[38]
3.5 Late 18th century
Benjamin Franklin.
Benjamin Franklin is frequently confused as the key lu-minary
behind electricity; William Watson and Ben-
jamin Franklin share the discovery of electrical poten-tials .
Benjamin Franklin promoted his investigations ofelectricity and
theories through the famous, though ex-tremely dangerous,
experiment of having his son y akite through a storm-threatened
sky. A key attached tothe kite string sparked and charged a Leyden
jar, thus es-tablishing the link between lightning and
electricity.*[39]Following these experiments, he invented a
lightning rod.It is either Franklin (more frequently) or Ebenezer
Kin-nersley of Philadelphia (less frequently) who is consid-ered to
have established the convention of positive andnegative
electricity.Theories regarding the nature of electricity were
quitevague at this period, and those prevalent were more orless
conicting. Franklin considered that electricity wasan imponderable
uid pervading everything, and which,in its normal condition, was
uniformly distributed in allsubstances. He assumed that the
electrical manifestationsobtained by rubbing glass were due to the
production ofan excess of the electric uid in that substance and
thatthe manifestations produced by rubbing wax were due toa decit
of the uid. This theory was opposed by RobertSymmer's Two-uidtheory
in 1759. By Symmer'stheory, the vitreous and resinous electricities
were re-garded as imponderable uids, each uid being composedof
mutually repellent particles while the particles of theopposite
electricities are mutually attractive. When thetwo uids unite as a
result of their attraction for one an-other, their eect upon
external objects is neutralized.The act of rubbing a body
decomposes the uids, one ofwhich remains in excess on the body and
manifests itselfas vitreous or resinous electricity.*[9]Up to the
time of Franklin's historic kite experi-ment,*[40] the identity of
the electricity developed byrubbing and by electrostatic machines
(frictional electric-ity) with lightning had not been generally
established.Dr. Wall,*[41] Abbot Nollet, Hauksbee,*[42]
StephenGray*[43] and John HenryWinkler*[44] had indeed sug-gested
the resemblance between the phenomena ofelec-tricityandlightning,
Gray having intimated that theyonly diered in degree. It was
doubtless Franklin, how-ever, who rst proposed tests to determine
the samenessof the phenomena. In a letter to Peter Comlinson of
Lon-don, on 19 October 1752, Franklin, referring to his
kiteexperiment, wrote,
At this key the phial (Leyden jar) maybe charged; and from the
electric re thus ob-tained spirits may be kindled, and all the
otherelectric experiments be formed which are usu-ally done by the
help of a rubbed glass globeor tube, and thereby the sameness of
the elec-tric matter with that of lightning be
completelydemonstrated.*[45]
On 10 May 1742 Thomas-Franois Dalibard, at Mar-ley (near Paris),
using a vertical iron rod 40 feet long,
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6 3 18TH CENTURY
obtained results corresponding to those recorded byFranklin and
somewhat prior to the date of Franklin'sexperiment. Franklin's
important demonstration of thesameness of frictional electricity
and lightning doubtlessadded zest to the eorts of the many
experimenters inthis eld in the last half of the 18th century, to
advancethe progress of the science.*[9]Franklin's observations
aided later scientists such asMichael Faraday, Luigi Galvani,
Alessandro Volta,Andr-Marie Ampre and Georg Simon Ohm,
whosecollective work provided the basis for modern electri-cal
technology and for whom fundamental units of elec-trical
measurement are named. Others who would ad-vance the eld of
knowledge included William Watson,Boze, Smeaton, Louis Guillaume Le
Monnier, Jacquesde Romas, Jean Jallabert, Giovanni Battista
Beccaria,Tiberius Cavallo, John Canton, Robert Symmer, AbbotNollet,
John Henry Winkler, Richman, Dr. Wilson,Kinnersley, Joseph
Priestley, Franz Aepinus, EdwardHussey Dlavai, Henry Cavendish and
Charles-Augustinde Coulomb. Descriptions of many of the
experimentsand discoveries of these early electrical scientists may
befound in the scientic publications of the time, notablythe
Philosophical Transactions, Philosophical Magazine,Cambridge
Mathematical Journal, Young's Natural Phi-losophy, Priestley's
History of Electricity, Franklin's Ex-periments and Observations on
Electricity, Cavalli's Trea-tise on Electricity and De la Rive's
Treatise on Electric-ity.*[9]Henry Elles was one of the rst people
to suggest linksbetween electricity and magnetism. In 1757 he
claimedthat he had written to the Royal Society in 1755 aboutthe
links between electricity and magnetism, assertingthatthere are
some things in the power of magnetismvery similar to those of
electricitybut he didnot byany means think them the same. In 1760
he simi-larly claimed that in 1750 he had been the rstto thinkhow
the electric re may be the cause of thunder.*[46]Among the more
important of the electrical research andexperiments during this
period were those of Franz Aepi-nus, a noted German scholar
(17241802) and HenryCavendish of London, England.*[9]Franz Aepinus
is credited as the rst to conceive of theview of the reciprocal
relationship of electricity and mag-netism. In his work Tentamen
Theoria Electricitatis etMagnetism,*[47] published in Saint
Petersburg in 1759,he gives the following amplication of Franklin's
theory,which in some of its features is measurably in accord
withpresent-day views:The particles of the electric uid re-pel each
other, attract and are attracted by the particles ofall bodies with
a force that decreases in proportion as thedistance increases; the
electric uid exists in the poresof bodies; it moves unobstructedly
through non-electric(conductors), but moves with diculty in
insulators; themanifestations of electricity are due to the unequal
distri-bution of the uid in a body, or to the approach of
bodiesunequally charged with the uid.Aepinus formulated a
corresponding theory of magnetism excepting that, in thecase of
magnetic phenomena, the uids only acted on theparticles of iron. He
also made numerous electrical ex-periments apparently showing that,
in order to manifestelectrical eects, tourmaline must be heated to
between37.5 and 100 C. In fact, tourmaline remains unelectri-ed
when its temperature is uniform, but manifests elec-trical
properties when its temperature is rising or falling.Crystals that
manifest electrical properties in this way aretermed pyroelectric;
along with tourmaline, these includesulphate of quinine and
quartz.*[9]Henry Cavendish independently conceived a theory
ofelectricity nearly akin to that of Aepinus.*[48] In 1784,he was
perhaps the rst to utilize an electric spark to pro-duce an
explosion of hydrogen and oxygen in the properproportions that
would create pure water. Cavendish alsodiscovered the inductive
capacity of dielectrics (insula-tors), and, as early as 1778,
measured the specic induc-tive capacity for beeswax and other
substances by com-parison with an air condenser.
Drawing of Coulomb's torsion balance. From Plate 13 of his1785
memoir.
Around 1784 C. A. Coulomb devised the torsion
balance,discovering what is now known as Coulomb's law: theforce
exerted between two small electried bodies variesinversely as the
square of the distance, not as Aepinus inhis theory of electricity
had assumed, merely inverselyas the distance. According to the
theory advanced byCavendish, the particles attract and are
attracted in-versely as some less power of the distance than the
cube.*[9] A large part of the domain of electricity became
virtu-ally annexed by Coulomb's discovery of the law of
inversesquares.
-
7Through the experiments of William Watson and othersproving
that electricity could be transmitted to a distance,the idea of
making practical use of this phenomenon be-gan, around 1753, to
engross theminds of inquisitive peo-ple. To this end, suggestions
as to the employment ofelectricity in the transmission of
intelligence were made.The rst of the methods devised for this
purpose wasprobably that of Georges Lesage in
1774.*[49]*[50]*[51]This method consisted of 24 wires, insulated
from one an-other and each having had a pith ball connected to its
dis-tant end. Each wire represented a letter of the alphabet.To
send a message, a desired wire was charged momen-tarily with
electricity from an electric machine, where-upon the pith ball
connected to that wire would y out.Other methods of telegraphing in
which frictional elec-tricity was employed were also tried, some of
which aredescribed in the history on the telegraph.*[9]The era of
galvanic or voltaic electricity represented arevolutionary break
from the historical focus on frictionalelectricity. Alessandro
Volta discovered that chemical re-actions could be used to create
positively charged anodesand negatively charged cathodes. When a
conductor wasattached between these, the dierence in the
electricalpotential (also known as voltage) drove a current
betweenthem through the conductor. The potential dierence be-tween
two points is measured in units of volts in recogni-tion of Volta's
work.*[9]The rst mention of voltaic electricity, although not
rec-ognized as such at the time, was probably made by JohannGeorg
Sulzer in 1767, who, upon placing a small disc ofzinc under his
tongue and a small disc of copper overit, observed a peculiar taste
when the respective metalstouched at their edges. Sulzer assumed
that when themet-als came together they were set into vibration,
acting uponthe nerves of the tongue to produce the eects noticed.In
1790, Prof. Luigi Alyisio Galvani of Bologna, whileconducting
experiments on "animal electricity", noticedthe twitching of a
frog's legs in the presence of an electricmachine. He observed that
a frog's muscle, suspended onan iron balustrade by a copper hook
passing through itsdorsal column, underwent lively convulsions
without anyextraneous cause, the electric machine being at this
timeabsent.*[9]To account for this phenomenon, Galvani assumed
thatelectricity of opposite kinds existed in the nerves andmuscles
of the frog, the muscles and nerves constitutingthe charged
coatings of a Leyden jar. Galvani publishedthe results of his
discoveries, together with his hypothe-sis, which engrossed the
attention of the physicists of thattime. The most prominent of
these was Volta, professorof physics at Pavia, who contended that
the results ob-served by Galvani were the result of the two metals,
cop-per and iron, acting as electromotors, and that the mus-cles of
the frog played the part of a conductor, completingthe circuit.
This precipitated a long discussion betweenthe adherents of the
conicting views. One group agreedwith Volta that the electric
current was the result of an
electromotive force of contact at the twometals; the
otheradopted a modication of Galvani's view and assertedthat the
current was the result of a chemical anity be-tween the metals and
the acids in the pile. Michael Fara-day wrote in the preface to his
Experimental Researches,relative to the question of whether
metallic contact is pro-ductive of a part of the electricity of the
voltaic pile: Isee no reason as yet to alter the opinion I have
given; ...but the point itself is of such great importance that I
in-tend at the rst opportunity renewing the inquiry, and, ifI can,
rendering the proofs either on the one side or theother, undeniable
to all.*[9]Even Faraday himself, however, did not settle the
con-troversy, and while the views of the advocates on bothsides of
the question have undergone modications, assubsequent
investigations and discoveries demanded, upto 1918 diversity of
opinion on these points continuedto crop out. Volta made numerous
experiments in sup-port of his theory and ultimately developed the
pile orbattery,*[52] which was the precursor of all
subsequentchemical batteries, and possessed the
distinguishingmeritof being the rst means by which a prolonged
continu-ous current of electricity was obtainable. Volta
commu-nicated a description of his pile to the Royal Society
ofLondon and shortly thereafter Nicholson and Cavendish(1780)
produced the decomposition of water by meansof the electric
current, using Volta's pile as the source ofelectromotive
force.*[9]
4 19th century
4.1 Early 19th century
Alessandro Volta.
In 1800 Alessandro Volta constructed the rst deviceto produce a
large electric current, later known as theelectric battery.
Napoleon, informed of his works, sum-moned him in 1801 for a
command performance of his
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8 4 19TH CENTURY
experiments. He received many medals and decorations,including
the Lgion d'honneur.Davy in 1806, employing a voltaic pile of
approximately250 cells, or couples, decomposed potash and soda,
show-ing that these substances were respectively the oxidesof
potassium and sodium, which metals previously hadbeen unknown.
These experiments were the beginningof electrochemistry, the
investigation of which Faradaytook up, and concerning which in 1833
he announced hisimportant law of electrochemical equivalents, viz.:
"Thesame quantity of electricitythat is, the same electric
cur-rentdecomposes chemically equivalent quantities of allthe
bodies which it traverses; hence the weights of elementsseparated
in these electrolytes are to each other as theirchemical
equivalents.Employing a battery of 2,000 el-ements of a voltaic
pile Humphry Davy in 1809 gave therst public demonstration of the
electric arc light, usingfor the purpose charcoal enclosed in a
vacuum.*[9]Somewhat important to note, it was not until many
yearsafter the discovery of the voltaic pile that the samenessof
annual and frictional electricity with voltaic electricitywas
clearly recognized and demonstrated. Thus as lateas January 1833 we
nd Faraday writing*[53] in a paperon the electricity of the
electric ray. "After an examina-tion of the experiments of
Walsh,*[54]*[55] Ingenhousz,Henry Cavendish, Sir H. Davy, and Dr.
Davy, no doubtremains on my mind as to the identity of the
electricity ofthe torpedo with common (frictional) and voltaic
electric-ity; and I presume that so little will remain on the mind
ofothers as to justify my refraining from entering at lengthinto
the philosophical proof of that identity. The doubtsraised by Sir
Humphry Davy have been removed by hisbrother, Dr. Davy; the results
of the latter being the re-verse of those of the former. ... The
general conclusionwhich must, I think, be drawn from this
collection of facts(a table showing the similarity, of properties
of the di-versely named electricities) is, that electricity,
whatevermay be its source, is identical in its nature.*[9]It is
proper to state, however, that prior to Faraday's timethe
similarity of electricity derived from dierent sourceswas more than
suspected. Thus, William Hyde Wol-laston,*[56] wrote in 1801:*[57]
"This similarity in themeans by which both electricity and
galvanism (voltaicelectricity) appear to be excited in addition to
the resem-blance that has been traced between their eects showsthat
they are both essentially the same and conrm anopinion that has
already been advanced by others, thatall the dierences discoverable
in the eects of the lat-ter may be owing to its being less intense,
but producedin much larger quantity.In the same paper
Wollastondescribes certain experiments in which he uses very newire
in a solution of sulphate of copper through which hepassed electric
currents from an electric machine. This isinteresting in connection
with the later day use of almostsimilarly arranged ne wires in
electrolytic receivers inwireless, or radio-telegraphy.*[9]
Hans Christian rsted.
In the rst half of the 19th century many very importantadditions
were made to the world's knowledge concerningelectricity and
magnetism. For example, in 1819 HansChristian rsted of Copenhagen
discovered the deect-ing eect of an electric current traversing a
wire upon- asuspended magnetic needle.*[9]This discovery gave a
clue to the subsequently provedintimate relationship between
electricity and magnetismwhich was promptly followed up by Ampre
who shortlythereafter (1821) announced his celebrated theory
ofelectrodynamics, relating to the force that one currentexerts
upon another, by its electro-magnetic eects,namely*[9]
1. Two parallel portions of a circuit attract one anotherif the
currents in them are owing in the same di-rection, and repel one
another if the currents ow inthe opposite direction.
2. Two portions of circuits crossing one anotherobliquely
attract one another if both the currentsow either towards or from
the point of crossing,and repel one another if one ows to and the
otherfrom that point.
3. When an element of a circuit exerts a force on an-other
element of a circuit, that force always tends tourge the second one
in a direction at right angles toits own direction.
Ampere brought a multitude of phenomena into theoryby his
investigations of the mechanical forces betweenconductors
supporting currents and magnets.
-
4.1 Early 19th century 9
The German physicist Seebeck discovered in 1821 thatwhen heat is
applied to the junction of twometals that hadbeen soldered together
an electric current is set up. This istermed Thermo-Electricity.
Seebeck's device consists ofa strip of copper bent at each end and
soldered to a plateof bismuth. A magnetic needle is placed parallel
with thecopper strip. When the heat of a lamp is applied to
thejunction of the copper and bismuth an electric current isset up
which deects the needle.*[9]Around this time, Simon Denis Poisson
attacked the dif-cult problem of induced magnetization, and his
results,though dierently expressed, are still the theory, as amost
important rst approximation. It was in the applica-tion of
mathematics to physics that his services to sciencewere performed.
Perhaps the most original, and certainlythe most permanent in their
inuence, were his memoirson the theory of electricity and
magnetism, which virtu-ally created a new branch of mathematical
physics.GeorgeGreen wroteAn Essay on the Application
ofMath-ematical Analysis to the Theories of Electricity and
Mag-netism in 1828. The essay introduced several importantconcepts,
among them a theorem similar to the modernGreen's theorem, the idea
of potential functions as cur-rently used in physics, and the
concept of what are nowcalled Green's functions. George Green was
the rst per-son to create a mathematical theory of electricity
andmagnetism and his theory formed the foundation for thework of
other scientists such as James Clerk Maxwell,William Thomson, and
others.Peltier in 1834 discovered an eect opposite to
Thermo-Electricity, namely, that when a current is passed througha
couple of dissimilar metals the temperature is loweredor raised at
the junction of the metals, depending on thedirection of the
current. This is termed the Peltieref-fect. The variations of
temperature are found to beproportional to the strength of the
current and not to thesquare of the strength of the current as in
the case of heatdue to the ordinary resistance of a conductor. This
sec-ond law is the C^2R law,*[58] discovered experimentallyin 1841
by the English physicist Joule. In other words,this important law
is that the heat generated in any partof an electric circuit is
directly proportional to the prod-uct of the resistance of this
part of the circuit and to thesquare of the strength of current
owing in the circuit.*[9]In 1822 Johann Schweigger devised the
rstgalvanometer. This instrument was subsequentlymuch improved by
Wilhelm Weber (1833). In 1825William Sturgeon of Woolwich, England,
invented thehorseshoe and straight bar electromagnet,
receivingtherefor the silver medal of the Society of Arts.*[59]In
1837 Carl Friedrich Gauss and Weber (both notedworkers of this
period) jointly invented a reectinggalvanometer for telegraph
purposes. This was the fore-runner of the Thomson reecting and
other exceedinglysensitive galvanometers once used in submarine
signalingand still widely employed in electrical measurements.
Arago in 1824 made the important discovery that when acopper
disc is rotated in its own plane, and if a magneticneedle be freely
suspended on a pivot over the disc, theneedle will rotate with the
disc. If on the other hand theneedle is xed it will tend to retard
the motion of the disc.This eect was termed Arago's
rotations.*[9]*[60]*[61]
Georg Simon Ohm.
Futile attempts were made by Charles Babbage, PeterBarlow, John
Herschel and others to explain this phe-nomenon. The true
explanation was reserved for Fara-day, namely, that electric
currents are induced in the cop-per disc by the cutting of the
magnetic lines of forceof the needle, which currents in turn react
on the nee-dle. Georg Simon Ohm did his work on resistance in
theyears 1825 and 1826, and published his results in 1827as the
book Die galvanische Kette, mathematisch bear-beitet.*[62]*[63] He
drew considerable inspiration fromFourier's work on heat conduction
in the theoretical ex-planation of his work. For experiments, he
initially usedvoltaic piles, but later used a thermocouple as this
pro-vided a more stable voltage source in terms of
internalresistance and constant potential dierence. He used
agalvanometer to measure current, and knew that the volt-age
between the thermocouple terminals was proportionalto the junction
temperature. He then added test wires ofvarying length, diameter,
and material to complete thecircuit. He found that his data could
be modeled througha simple equation with variable composed of the
read-ing from a galvanometer, the length of the test conduc-tor,
thermocouple junction temperature, and a constantof the entire
setup. From this, Ohm determined his lawof proportionality and
published his results. In 1827, heannounced the now famous law that
bears his name, that
-
10 4 19TH CENTURY
is:
Electromotive force = Current Resistance*[64]
Ohm brought into order a host of puzzling facts connect-ing
electromotive force and electric current in conduc-tors, which all
previous electricians had only succeeded inloosely binding together
qualitatively under some rathervague statements. Ohm found that the
results could besummed up in such a simple law and by Ohm's
discoverya large part of the domain of electricity became annexedto
theory.
4.2 Faraday and Henry
Joseph Henry.
The discovery of electromagnetic induction was made al-most
simultaneously, although independently, byMichaelFaraday, who was
rst to make the discovery in 1831,and Joseph Henry in
1832.*[65]*[66] Henry's discoveryof self-induction and his work on
spiral conductors usinga copper coil were made public in 1835, just
before thoseof Faraday.*[67]*[68]*[69]In 1831 began the
epoch-making researches of MichaelFaraday, the famous pupil and
successor of HumphryDavy at the head of the Royal Institution,
London, re-lating to electric and electromagnetic induction. The
re-markable researches of Faraday, the prince of experimen-talists,
on electrostatics and electrodynamics and the in-duction of
currents. These were rather long in beingbrought from the crude
experimental state to a compactsystem, expressing the real essence.
Faraday was not
a competent mathematician,*[70]*[71]*[72] but had hebeen one, he
would have been greatly assisted in his re-searches, have saved
himself much useless speculation,and would have anticipated much
later work. He would,for instance, knowing Ampere's theory, by his
own re-sults have readily been led to Neumann's theory, and
theconnected work of Helmholtz and Thomson. Faraday'sstudies and
researches extended from 1831 to 1855 anda detailed description of
his experiments, deductions andspeculations are to be found in his
compiled papers, en-titled Experimental Researches in Electricity.'
Faradaywas by profession a chemist. He was not in the
remotestdegree a mathematician in the ordinary senseindeed itis a
question if in all his writings there is a single mathe-matical
formula.*[9]
Michael Faraday.
The experiment which led Faraday to the discovery ofelectric
induction was made as follows: He constructedwhat is now and was
then termed an induction coil, theprimary and secondary wires of
which were wound on awooden bobbin, side by side, and insulated
from one an-other. In the circuit of the primary wire he placed a
bat-tery of approximately 100 cells. In the secondary wire
heinserted a galvanometer. On making his rst test he ob-served no
results, the galvanometer remaining quiescent,but on increasing the
length of the wires he noticed a de-ection of the galvanometer in
the secondary wire whenthe circuit of the primary wire was made and
broken.This was the rst observed instance of the developmentof
electromotive force by electromagnetic induction.*[9]He also
discovered that induced currents are establishedin a second closed
circuit when the current strength is var-
-
4.3 Middle 19th century 11
ied in the rst wire, and that the direction of the currentin the
secondary circuit is opposite to that in the rst cir-cuit. Also
that a current is induced in a secondary cir-cuit when another
circuit carrying a current is moved toand from the rst circuit, and
that the approach or with-drawal of a magnet to or from a closed
circuit inducesmomentary currents in the latter. In short, within
thespace of a few months Faraday discovered by experi-ment
virtually all the laws and facts now known concern-ing
electro-magnetic induction and magneto-electric in-duction. Upon
these discoveries, with scarcely an ex-ception, depends the
operation of the telephone, thedynamo machine, and incidental to
the dynamo electricmachine practically all the gigantic electrical
industriesof the world, including electric lighting, electric
traction,the operation of electric motors for power purposes,
andelectro-plating, electrotyping, etc.*[9]In his investigations of
the peculiar manner in which ironlings arrange themselves on a
cardboard or glass in prox-imity to the poles of a magnet, Faraday
conceived the ideaof magnetic "lines of force" extending from pole
to poleof the magnet and along which the lings tend to
placethemselves. On the discovery being made that magneticeects
accompany the passage of an electric current in awire, it was also
assumed that similar magnetic lines offorce whirled around the
wire. For convenience and toaccount for induced electricity it was
then assumed thatwhen these lines of force are "cut" by a wire in
passingacross them or when the lines of force in rising and
fallingcut the wire, a current of electricity is developed, or to
bemore exact, an electromotive force is developed in thewire that
sets up a current in a closed circuit. Faradayadvanced what has
been termed the molecular theory ofelectricity*[73] which assumes
that electricity is the man-ifestation of a peculiar condition of
the molecule of thebody rubbed or the ether surrounding the body.
Fara-day also, by experiment, discovered paramagnetism
anddiamagnetism, namely, that all solids and liquids are ei-ther
attracted or repelled by a magnet. For example, iron,nickel,
cobalt, manganese, chromium, etc., are paramag-netic (attracted by
magnetism), whilst other substances,such as bismuth, phosphorus,
antimony, zinc, etc., arerepelled by magnetism or are
diamagnetic.*[9]*[74]Brugans of Leyden in 1778 and Le Baillif and
Becquerelin 1827*[75] had previously discovered diamagnetism inthe
case of bismuth and antimony. Faraday also rediscov-ered specic
inductive capacity in 1837, the results of theexperiments by
Cavendish not having been published atthat time. He also
predicted*[76] the retardation of sig-nals on long submarine cables
due to the inductive eectof the insulation of the cable, in other
words, the staticcapacity of the cable.*[9]The 25 years immediately
following Faraday's discover-ies of electric induction were
fruitful in the promulgationof laws and facts relating to induced
currents and to mag-netism. In 1834 Heinrich Lenz and Moritz von
Jacobi in-dependently demonstrated the now familiar fact that
the
currents induced in a coil are proportional to the numberof
turns in the coil. Lenz also announced at that time hisimportant
law that, in all cases of electromagnetic induc-tion the induced
currents have such a direction that theirreaction tends to stop the
motion that produces them, alaw that was perhaps deducible from
Faraday's explana-tion of Arago's rotations.*[9]*[77]The induction
coil was rst designed by Nicholas Callanin 1836. In 1845 Joseph
Henry, the American physicist,published an account of his valuable
and interesting ex-periments with induced currents of a high order,
showingthat currents could be induced from the secondary of
aninduction coil to the primary of a second coil, thence toits
secondary wire, and so on to the primary of a thirdcoil, etc.*[78]
Heinrich Daniel Ruhmkor further devel-oped the induction coil, the
Ruhmkor coil was patentedin 1851,*[79] and he utilized long
windings of copperwire to achieve a spark of approximately 2 inches
(50mm) in length. In 1857, after examining a greatly im-proved
version made by an American inventor, EdwardSamuel
Ritchie,*[80]*[81] Ruhmkor improved his de-sign (as did other
engineers), using glass insulation andother innovations to allow
the production of sparks morethan 300 millimetres (12 in)
long.*[82]
4.3 Middle 19th centuryUp to the middle of the 19th century,
indeed up to about1870, electrical science was, it may be said, a
sealedbook to the majority of electrical workers. Prior tothis time
a number of handbooks had been publishedon electricity and
magnetism, notably Auguste de LaRive's exhaustive ' Treatise on
Electricity,'*[84] in 1851(French) and 1853 (English); August
Beer's Einleitungin die Elektrostatik, die Lehre vom Magnetismus
und dieElektrodynamik,*[85] Wiedemann's ' Galvanismus,'
andReiss'*[86] 'Reibungsal-elektricitat.' But these works
con-sisted in the main in details of experiments with elec-tricity
and magnetism, and but little with the laws andfacts of those
phenomena. Henry d'Abria*[87]*[88]published the results of some
researches into the lawsof induced currents, but owing to their
complexity ofthe investigation it was not productive of very
notableresults.*[89] Around the mid-19th century, FleemingJenkin's
work on ' Electricity and Magnetism*[90] ' andClerk Maxwell's '
Treatise on Electricity and Magnetism 'were published.*[9]These
books were departures from the beaten path. AsJenkin states in the
preface to his work the science ofthe schools was so dissimilar
from that of the practicalelectrician that it was quite impossible
to give studentssucient, or even approximately sucient, textbooks.
Astudent he said might have mastered de la Rive's large andvaluable
treatise and yet feel as if in an unknown coun-try and listening to
an unknown tongue in the companyof practical men. As another writer
has said, with thecoming of Jenkin's and Maxwell's books all
impediments
-
12 4 19TH CENTURY
in the way of electrical students were removed, "the fullmeaning
of Ohm's law becomes clear; electromotive force,dierence of
potential, resistance, current, capacity, linesof force,
magnetization and chemical anity were mea-surable, and could be
reasoned about, and calculationscould be made about them with as
much certainty as cal-culations in dynamics".*[9]*[91]About 1850,
Kirchho published his laws relating tobranched or divided circuits.
He also showed mathe-matically that according to the then
prevailing electro-dynamic theory, electricity would be propagated
alonga perfectly conducting wire with the velocity of
light.Helmholtz investigated mathematically the eects of in-duction
upon the strength of a current and deducedtherefrom equations,
which experiment conrmed, show-ing amongst other important points
the retarding eectof self-induction under certain conditions of the
cir-cuit.*[9]*[92]
Sir William Thomson.
In 1853, Sir William Thomson (later Lord Kelvin) pre-dicted as a
result of mathematical calculations the oscil-latory nature of the
electric discharge of a condenser cir-cuit. To Henry, however,
belongs the credit of discern-ing as a result of his experiments in
1842 the oscillatorynature of the Leyden jar discharge. He
wrote:*[93] Thephenomena require us to admit the existence of a
princi-pal discharge in one direction, and then several reex
ac-tions backward and forward, each more feeble than thepreceding,
until the equilibrium is obtained. These oscil-lations were
subsequently observed by B. W. Feddersen(1857)*[94]*[95] who using
a rotating concave mirrorprojected an image of the electric spark
upon a sensi-tive plate, thereby obtaining a photograph of the
spark
which plainly indicated the alternating nature of the
dis-charge. Sir William Thomson was also the discoverer ofthe
electric convection of heat (theThomsoneect).He designed for
electrical measurements of precision hisquadrant and absolute
electrometers. The reecting gal-vanometer and siphon recorder, as
applied to submarinecable signaling, are also due to him.*[9]About
1876 the American physicist Henry AugustusRowland of Baltimore
demonstrated the important factthat a static charge carried around
produces the samemagnetic eects as an electric current.*[96]*[97]
The Im-portance of this discovery consists in that it may aorda
plausible theory of magnetism, namely, that magnetismmay be the
result of directed motion of rows of moleculescarrying static
charges.*[9]After Faraday's discovery that electric currents could
bedeveloped in a wire by causing it to cut across the linesof force
of a magnet, it was to be expected that attemptswould be made to
construct machines to avail of this factin the development of
voltaic currents.*[98] The rst ma-chine of this kind was due to
Hippolyte Pixii, 1832. Itconsisted of two bobbins of iron wire,
opposite whichthe poles of a horseshoe magnet were caused to
rotate.As this produced in the coils of the wire an
alternatingcurrent, Pixii arranged a commutating device
(commuta-tor) that converted the alternating current of the coils
orarmature into a direct current in the external circuit.
Thismachine was followed by improved forms of magneto-electric
machines due to RItchie, Saxton, Clarke 1834,Stohrer 1843, Nollet
1849, Shepperd 1856, VanMaldern,Siemens, Wilde and others.*[9]A
notable advance in the art of dynamo constructionwas made by Mr. S.
A. Varley in 1866*[99] and byDr. Charles William Siemens and Mr.
Charles Wheat-stone,*[100] who independently discovered that when
acoil of wire, or armature, of the dynamo machine is ro-tated
between the poles (or in theeld) of an electro-magnet, a weak
current is set up in the coil due to resid-ual magnetism in the
iron of the electromagnet, and thatif the circuit of the armature
be connected with the cir-cuit of the electromagnet, the weak
current developed inthe armature increases the magnetism in the
eld. Thisfurther increases the magnetic lines of force in which
thearmature rotates, which still further increases the currentin
the electromagnet, thereby producing a correspondingincrease in the
eld magnetism, and so on, until the max-imum electromotive force
which the machine is capableof developing is reached. By means of
this principle thedynamomachine develops its own magnetic eld,
therebymuch increasing its eciency and economical operation.Not by
any means, however, was the dynamo electric ma-chine perfected at
the time mentioned.*[9]In 1860 an important improvement had been
made byDr. Antonio Pacinotti of Pisa who devised the rst elec-tric
machine with a ring armature. This machine wasrst used as an
electric motor, but afterward as a gener-
-
4.4 Maxwell 13
ator of electricity. The discovery of the principle of
thereversibility of the dynamo electric machine
(variouslyattributed to Walenn 1860; Pacinotti 1864 ;
Fontaine,Gramme 1873; Deprez 1881, and others) whereby it maybe
used as an electric motor or as a generator of electric-ity has
been termed one of the greatest discoveries of the19th
century.*[9]In 1872 the drum armature was devised by
Hefner-Alteneck. This machine in a modied form was subse-quently
known as the Siemens dynamo. These machineswere presently followed
by the Schuckert, Gulcher,*[101]Fein,*[102]*[103] Brush,
Hochhausen, Edison and thedynamo machines of numerous other
inventors. In theearly days of dynamo machine construction the
machineswere mainly arranged as direct current generators,
andperhaps the most important application of such machinesat that
time was in electro-plating, for which purpose ma-chines of low
voltage and large current strength were
em-ployed.*[9]*[104]Beginning about 1887 alternating current
generatorscame into extensive operation and the commercial
devel-opment of the transformer, by means of which currentsof low
voltage and high current strength are transformedto currents of
high voltage and low current strength, andvice versa, in time
revolutionized the transmission ofelectric power to long distances.
Likewise the intro-duction of the rotary converter (in connection
with thestep-downtransformer) which converts alternating cur-rents
into direct currents (and vice versa) has eectedlarge economies in
the operation of electric power sys-tems.*[9]*[105]Before the
introduction of dynamo electric machines,voltaic, or primary,
batteries were extensively used forelectro-plating and in
telegraphy. There are two distincttypes of voltaic cells, namely,
theopenand theclosed, or constant, type. The open type in brief is
thattype which operated on closed circuit becomes, after ashort
time, polarized; that is, gases are liberated in thecell which
settle on the negative plate and establish a re-sistance that
reduces the current strength. After a briefinterval of open circuit
these gases are eliminated or ab-sorbed and the cell is again ready
for operation. Closedcircuit cells are those in which the gases in
the cells areabsorbed as quickly as liberated and hence the output
ofthe cell is practically uniform. The Leclanch and Daniellcells,
respectively, are familiar examples of theopenand closedtype of
voltaic cell. The opencellsare used very extensively at present,
especially in the drycell form, and in annunciator and other open
circuit sig-nal systems. Batteries of the Daniell orgravitytypewere
employed almost generally in the United States andCanada as the
source of electromotive force in telegra-phy before the dynamo
machine became available, andstill are largely used for this
service or aslocalcells.Batteries of thegravityand the
Edison-Lalande typesare still much used inclosed
circuitsystems.*[9]
In the late 19th century, the term luminiferous aether,meaning
light-bearing aether, was a conjectured mediumfor the propagation
of light.*[106] The word aether stemsvia Latin from the Greek ,
from a root meaning tokindle, burn, or shine. It signies the
substance whichwas thought in ancient times to ll the upper regions
ofspace, beyond the clouds.
4.4 Maxwell
James Clerk Maxwell.
In 1864 James Clerk Maxwell of Edinburgh announcedhis
electromagnetic theory of light, which was perhapsthe greatest
single step in the world's knowledge of elec-tricity.*[107] Maxwell
had studied and commented onthe eld of electricity and magnetism as
early as 1855/6when On Faraday's lines of force*[108] was read to
theCambridge Philosophical Society. The paper presenteda simplied
model of Faraday's work, and how the twophenomena were related. He
reduced all of the cur-rent knowledge into a linked set of
dierential equa-tions with 20 equations in 20 variables. This work
waslater published as On Physical Lines of Force in
March1861.*[109] In order to determine the force which is act-ing
on any part of the machine we must nd its momen-tum, and then
calculate the rate at which this momentumis being changed. This
rate of change will give us theforce. The method of calculation
which it is necessary toemploy was rst given by Lagrange, and
afterwards devel-oped, with some modications, by Hamilton's
equations.It is usually referred to as Hamilton's principle; when
theequations in the original form are used they are knownas
Lagrange's equations. Now Maxwell logically showedhow these methods
of calculation could be applied to the
-
14 4 19TH CENTURY
electro-magnetic eld.*[110] The energy of a dynamicalsystem is
partly kinetic, partly potential. Maxwell sup-poses that the
magnetic energy of the eld is kinetic en-ergy, the electric energy
potential.*[111]Around 1862, while lecturing at King's College,
Maxwellcalculated that the speed of propagation of an
electro-magnetic eld is approximately that of the speed of light.He
considered this to be more than just a coincidence,and commented
"We can scarcely avoid the conclusionthat light consists in the
transverse undulations of the samemedium which is the cause of
electric and magnetic phe-nomena."*[112]Working on the problem
further, Maxwell showed thatthe equations predict the existence of
waves of oscillat-ing electric and magnetic elds that travel
through emptyspace at a speed that could be predicted from simple
elec-trical experiments; using the data available at the
time,Maxwell obtained a velocity of 310,740,000 m/s. In his1864
paper A Dynamical Theory of the ElectromagneticField, Maxwell
wrote, The agreement of the results seemsto show that light and
magnetism are aections of the samesubstance, and that light is an
electromagnetic disturbancepropagated through the eld according to
electromagneticlaws.*[113]As already noted herein Faraday, and
before him, Am-pre and others, had inklings that the luminiferous
etherof space was also the medium for electric action. It wasknown
by calculation and experiment that the velocity ofelectricity was
approximately 186,000 miles per second;that is, equal to the
velocity of light, which in itself sug-gests the idea of a
relationship between -electricity andlight.A number of the earlier
philosophers or mathe-maticians, as Maxwell terms them, of the 19th
century,held the view that electromagnetic phenomena were
ex-plainable by action at a distance. Maxwell, followingFaraday,
contended that the seat of the phenomena wasin the medium. The
methods of the mathematicians inarriving at their results were
synthetical while Faraday'smethods were analytical. Faraday in his
mind's eye sawlines of force traversing all space where the
mathemati-cians saw centres of force attracting at a distance.
Faradaysought the seat of the phenomena in real actions going onin
the medium; they were satised that they had foundit in a power of
action at a distance on the electric u-ids.*[114]Both of these
methods, as Maxwell points out, had suc-ceeded in explaining the
propagation of light as an elec-tromagnetic phenomenon while at the
same time the fun-damental conceptions of what the quantities
concernedare, radically diered. The mathematicians assumed
thatinsulators were barriers to electric currents; that, for
in-stance, in a Leyden jar or electric condenser the electric-ity
was accumulated at one plate and that by some occultaction at a
distance electricity of an opposite kind wasattracted to the other
plate.Maxwell, looking further than Faraday, reasoned that if
light is an electromagnetic phenomenon and is transmis-sible
through dielectrics such as glass, the phenomenonmust be in the
nature of electromagnetic currents in thedielectrics. He therefore
contended that in the chargingof a condenser, for instance, the
action did not stop atthe insulator, but that
somedisplacementcurrents areset up in the insulating medium, which
currents continueuntil the resisting force of the medium equals
that of thecharging force. In a closed conductor circuit, an
electriccurrent is also a displacement of electricity.The conductor
oers a certain resistance, akin to friction,to the displacement of
electricity, and heat is developedin the conductor, proportional to
the square of the cur-rent(as already stated herein), which current
ows as longas the impelling electric force continues. This
resistancemay be likened to that met with by a ship as it
displacesin the water in its progress. The resistance of the
dielec-tric is of a dierent nature and has been compared to
thecompression of multitudes of springs, which, under com-pression,
yield with an increasing back pressure, up to apoint where the
total back pressure equals the initial pres-sure. When the initial
pressure is withdrawn the energyexpended in compressing the
springsis returned tothe circuit, concurrently with the return of
the springs totheir original condition, this producing a reaction
in theopposite direction. Consequently the current due to
thedisplacement of electricity in a conductor may be contin-uous,
while the displacement currents in a dielectric aremomentary and,
in a circuit or medium which containsbut little resistance compared
with capacity or inductancereaction, the currents of discharge are
of an oscillatory oralternating nature.*[115]Maxwell extended this
view of displacement currents indielectrics to the ether of free
space. Assuming light to bethe manifestation of alterations of
electric currents in theether, and vibrating at the rate of light
vibrations, thesevibrations by induction set up corresponding
vibrations inadjoining portions of the ether, and in this way the
undu-lations corresponding to those of light are propagated asan
electromagnetic eect in the ether. Maxwell's electro-magnetic
theory of light obviously involved the existenceof electric waves
in free space, and his followers set them-selves the task of
experimentally demonstrating the truthof the theory. By 1871, he
presented the Remarks on themathematical classication of physical
quantities.*[116]
4.5 End of the 19th centuryIn 1887, the German physicist
Heinrich Hertz ina series of experiments proved the actual
existenceelectromagnetic waves, showing that transverse free
spaceelectromagnetic waves can travel over some distance
aspredicted by Maxwell and Faraday. Hertz published hiswork in a
book titled: Electric waves: being researcheson the propagation of
electric action with nite velocitythrough space.*[117] The
discovery of electromagneticwaves in space led to the development
in the closing years
-
4.5 End of the 19th century 15
Heinrich Hertz.
of the 19th century of radio.The electron as a unit of charge in
electrochemistry wasposited by G. Johnstone Stoney in 1874, who
also coinedthe term electron in 1894. Plasma was rst identied in
aCrookes tube, and so described by Sir William Crookesin 1879 (he
called itradiant matter).*[118] The placeof electricity in leading
up to the discovery of those beau-tiful phenomena of the Crookes
Tube (due to Sir WilliamCrookes), viz., Cathode rays,*[119] and
later to the dis-covery of Roentgen or X-rays, must not be
overlooked,since without electricity as the excitant of the tube
thediscovery of the rays might have been postponed inde-nitely. It
has been noted herein that Dr. William Gilbertwas termed the
founder of electrical science. This must,however, be regarded as a
comparative statement.*[9]Oliver Heaviside was a self-taught
scholar who reformu-lated Maxwell's eld equations in terms of
electric andmagnetic forces and energy ux, and independently
co-formulated vector analysis. His series of articles con-tinued
the work entitled "Electromagnetic Induction andits Propagation",
commenced in The Electrician in 1885to nearly 1887 (ed., the latter
part of the work deal-ing with the propagation of electromagnetic
waves alongwires through the dielectric surrounding them), when
thegreat pressure on space and the want of readers appearedto
necessitate its abrupt discontinuance.*[120] (A strag-gler piece
appeared December 31, 1887.) He wrote aninterpretation of the
transcendental formulae of electro-magnetism. Following the real
object of true natural-ists*[121] when they employ mathematics to
assist them,he wrote to nd out the connections of known phenom-ena,
and by deductive reasoning, to obtain a knowledgeof electromagnetic
phenomena. Although at odds with
1Oliver Heaviside.
the scientic establishment for most of his life,
Heavisidechanged the face of mathematics and science for years
tocome.Of the changes in the eld of electromagnetic theory,
cer-tain conclusions from Electro-Magnetic Theory*[122] byHeaviside
are, if not drawn, at least indicated in this book.Two of them may
be stated as follows:
1. That magnetism is a phenomenon of motion and nota statical
phenomenon; also that this motion is morelikely to be translational
than vortical.
2. That all electric currents are phenomena consequentupon the
emission of electro-magnetic wave distur-bances in the aether, and
that the proper treatmentof all the phenomena of currents and
magnetic uxshould be considered as the consequence, and not asthe
cause, of electro-magnetic waves.
The ultimate results of his work are twofold. (1) The
rstultimate result is purely mathematical, which is impor-tant only
to those who study mathematical physics. Thesystem of vectorial
algebra*[123] as developed by Mr.Heaviside was used because of ease
for physical inves-tigations to the methods of quaternions. (2) The
sec-ond ultimate result is physical. It consists in more
closelyuniting the more recondite problems of telegraphy,
tele-phony, Teslaic phenomena and Hertzian phenomena withthe
fundamental properties of the aether. In elucidat-ing this
connection, the merit of the book appears mostprominently as a
stepping-stone to the goal in the full viewof all physical
analysis, namely, the resolution of all phys-
-
16 4 19TH CENTURY
ical phenomena to the activities of the aether, and of mat-ter
in the aether, under the laws of dynamics.*[124]During the late
1890s a number of physicists proposedthat electricity, as observed
in studies of electrical con-duction in conductors, electrolytes,
and cathode ray tubes,consisted of discrete units, which were given
a variety ofnames, but the reality of these units had not been
con-rmed in a compelling way. However, there were alsoindications
that the cathode rays had wavelike proper-ties.*[9]Faraday, Weber,
Helmholtz, Cliord and others hadglimpses of this view; and the
experimental works ofZeeman, Goldstein, Crookes, J. J. Thomson and
othershad greatly strengthened this view. Weber predicted
thatelectrical phenomena were due to the existence of elec-trical
atoms, the inuence of which on one another de-pended on their
position and relative accelerations and ve-locities. Helmholtz and
others also contended that the ex-istence of electrical atoms
followed from Faraday's lawsof electrolysis, and Johnstone Stoney,
to whom is due thetermelectron, showed that each chemical ion of
thedecomposed electrolyte carries a denite and constantquantity of
electricity, and inasmuch as these chargedions are separated on the
electrodes as neutral substancesthere must be an instant, however
brief, when the chargesmust be capable of existing separately as
electrical atoms;while in 1887, Cliord wrote:There is great reason
tobelieve that every material atom carries upon it a smallelectric
current, if it does not wholly consist of this cur-rent.*[9]
Nikola Tesla, c. 1896.
The Serbian American engineer Nikola Tesla learned of
Hertzexperiments at the Exposition Universelle in 1889and
launched into his own experiments in high frequencyand high
potential current developinghigh-frequencyalternators (which
operated around 15,000 hertz).*[125].He concluded from his
observations that Maxwell andHertz were wrong about the existence
of airborne electro-magnetic waves (which he attributed it to what
he calledelectrostatic thrusts)*[126] but saw great potential
inMaxwell's idea that that electricity and light were part ofthe
same phenomena, seeing it as a way to create a newtype of wireless
electric lighting.*[127] By 1893 he wasgiving lectures on "On Light
and Other High FrequencyPhenomena", including a demonstration where
he wouldlight a Geissler tubes wirelessly. Tesla worked for
manyyears after that trying to develop a wireless power
distri-bution system.*[128]
J.J. Thomson.
In 1896, J.J. Thomson performed experiments indicatingthat
cathode rays really were particles, found an accuratevalue for
their charge-to-mass ratio e/m, and found thate/m was independent
of cathode material. He made goodestimates of both the charge e and
the mass m, ndingthat cathode ray particles, which he
calledcorpuscles,had perhaps one thousandth of the mass of the
least mas-sive ion known (hydrogen). He further showed that
thenegatively charged particles produced by radioactive ma-terials,
by heated materials, and by illuminated materials,were universal.
The nature of the Crookes tube "cathoderay" matter was identied by
Thomson in 1897.*[129]In the late 19th century, the MichelsonMorley
experi-ment was performed by Albert A. Michelson and EdwardW.
Morley at what is now Case Western Reserve Univer-sity. It is
generally considered to be the evidence against
-
4.6 Second Industrial Revolution 17
the theory of a luminiferous aether. The experiment hasalso been
referred to as the kicking-o point for thetheoretical aspects of
the Second Scientic Revolution.*[130] Primarily for this work,
Michelson was awardedthe Nobel Prize in 1907. Dayton Miller
continued withexperiments, conducting thousands of measurements
andeventually developing the most accurate interferometer inthe
world at that time. Miller and others, such as Mor-ley, continue
observations and experiments dealing withthe concepts.*[131] A
range of proposed aether-draggingtheories could explain the null
result but these were morecomplex, and tended to use
arbitrary-looking coecientsand physical assumptions.*[9]By the end
of the 19th century electrical engineers hadbecome a distinct
profession, separate from physicistsand inventors. They created
companies that investigated,developed and perfected the techniques
of electricitytransmission, and gained support from governments
allover the world for starting the rst worldwide
electricaltelecommunication network, the telegraph network.
Pio-neers in this eld included Werner von Siemens, founderof
Siemens AG in 1847, and John Pender, founder ofCable &
Wireless.The rst public demonstration of aalternator systemtook
place in 1886. Large two-phase alternating currentgenerators were
built by a British electrician, J.E.H. Gor-don,*[132] in 1882. Lord
Kelvin and Sebastian Ferrantialso developed early alternators,
producing frequenciesbetween 100 and 300 hertz. After 1891,
polyphase alter-nators were introduced to supply currents of
multiple dif-fering phases.*[133] Later alternators were designed
forvarying alternating-current frequencies between sixteenand about
one hundred hertz, for use with arc lighting,incandescent lighting
and electric motors.*[134]The possibility of obtaining the electric
current in largequantities, and economically, by means of dynamo
elec-tric machines gave impetus to the development of incan-descent
and arc lighting. Until these machines had at-tained a commercial
basis voltaic batteries were the onlyavailable source of current
for electric lighting and power.The cost of these batteries,
however, and the dicul-ties of maintaining them in reliable
operation were pro-hibitory of their use for practical lighting
purposes. Thedate of the employment of arc and incandescent
lampsmay be set at about 1877.*[9]Even in 1880, however, but little
headway had been madetoward the general use of these illuminants;
the rapid sub-sequent growth of this industry is a matter of
generalknowledge.*[135] The employment of storage batteries,which
were originally termed secondary batteries or ac-cumulators, began
about 1879. Such batteries are nowutilized on a large scale as
auxiliaries to the dynamo ma-chine in electric power-houses and
substations, in electricautomobiles and in immense numbers in
automobile ig-nition and starting systems, also in re alarm
telegraphyand other signal systems.*[9]
World's Fair Tesla presentation.
In 1893, the World's Columbian International Expositionwas held
in a building which was devoted to electricalexhibits. General
Electric Company (backed by Edisonand J.P. Morgan) had proposed to
power the electric ex-hibits with direct current at the cost of one
million dol-lars. However, Westinghouse proposed to illuminate
theColumbian Exposition in Chicago with alternating cur-rent for
half that price, and Westinghouse won the bid.It was an historical
moment and the beginning of a revo-lution, as George Westinghouse
introduced the public toelectrical power by illuminating the
Exposition.
4.6 Second Industrial RevolutionMain article: Second Industrial
RevolutionBetween 1885 and 1890 Galileo Ferraris in Italy,
Thomas Edison.
-
18 4 19TH CENTURY
Nikola Tesla in the United States, and Mikhail
Dolivo-Dobrovolsky in Germany explored poly-phase currentscombined
with electromagnetic induction leading to thedevelopment of
practical AC induction motors.*[136]The AC induction motor helped
usher in the Second In-dustrial Revolution. The rapid advance of
electrical tech-nology in the latter 19th and early 20th centuries
led tocommercial rivalries. In the War of Currents in the
late1880s, George Westinghouse and Thomas Edison be-came
adversaries due to Edison's promotion of direct cur-rent (DC) for
electric power distribution over alternatingcurrent (AC) advocated
by Westinghouse.Several inventors helped develop commercial
systems.Samuel Morse, inventor of a long-range telegraph;Thomas
Edison, inventor of the rst commercial electri-cal energy
distribution network; George Westinghouse,inventor of the electric
locomotive; Alexander GrahamBell, the inventor of the telephone and
founder of a suc-cessful telephone business.In 1871 the electric
telegraph had grown to large pro-portions and was in use in every
civilized country in theworld, its lines forming a network in all
directions overthe surface of the land. The system most generally
in usewas the electromagnetic telegraph due to S. F. B. Morseof New
York, or modications of his system.*[137] Sub-marine cables*[138]
connecting the Eastern and West-ern hemispheres were also in
successful operation at thattime.*[9]When, however, in 1918 one
views the vast applicationsof electricity to electric light,
electric railways, electricpower and other purposes (all it may be
repeated madepossible and practicable by the perfection of the
dynamomachine), it is dicult to believe that no longer ago than1871
the author of a book published in that year, in re-ferring to the
state of the art of applied electricity at thattime, could have
truthfully written:The most importantand remarkable of the uses
which have beenmade of elec-tricity consists in its application to
telegraph purposes.*[139] The statement was, however, quite
accurate andperhaps the time could have been carried forward to
theyear 1876 without material modication of the remarks.In that
year the telephone, due to Alexander GrahamBell,was invented, but
it was not until several years thereafterthat its commercial
employment began in earnest. Sincethat time also the sister
branches of electricity just men-tioned have advanced and are
advancing with such gigan-tic strides in every direction that it is
dicult to place alimit upon their progress. Electrical devices
account ofthe use of electricity in the arts and industries.*[9]AC
replaced DC for central station power generation andpower
distribution, enormously extending the range andimproving the
safety and eciency of power distribution.Edison's low-voltage
distribution system using DC ulti-mately lost to AC devices
proposed by others: Westing-house' AC system, Tesla's AC
inventions, and the theo-retical work of Charles Proteus Steinmetz.
The success-
Charles Proteus Steinmetz, theoretician of alternating
current.
ful Niagara Falls system was a turning point in the ac-ceptance
of alternating current. Eventually, the GeneralElectric company
(formed by a merger between Edison'scompanies and the AC-based
rival Thomson-Houston)began manufacture of AC machines. Centralized
powergeneration became possible when it was recognized
thatalternating current electric power lines can transport
elec-tricity at low costs across great distances by taking
advan-tage of the ability to change voltage across the
distributionpath using power transformers. The voltage is raised
atthe point of generation (a representative number is a gen-erator
voltage in the low kilovolt range) to a much highervoltage (tens of
thousands to several hundred thousandvolts) for primary
transmission, followed to several down-ward transformations, to as
low as that used in residentialdomestic use.*[9]The International
Electro-Technical Exhibition of 1891featuring the long distance
transmission of high-power,three-phase electric current. It was
held between 16 Mayand 19 October on the disused site of the three
formerWestbahnhfe(Western Railway Stations) in Frankfurtam Main.
The exhibition featured the rst long distancetransmission of
high-power, three-phase electric current,which was generated 175 km
away at Lauen amNeckar.As a result of this successful eld trial,
three-phase cur-rent became established for electrical transmission
net-works throughout the world.*[9]Much was done in the direction
in the improvement ofrailroad terminal facilities, and it is dicult
to nd onesteam railroad engineer who would have denied that allthe
important steam railroads of this country were not tobe operated
electrically. In other directions the progressof events as to the
utilization of electric power was ex-pected to be equally rapid. In
every part of the worldthe power of falling water, nature's
perpetual motion ma-chine, which has been going to waste since the
world be-
-
5.1 Lorentz and Poincar 19
gan, is now being converted into electricity and transmit-ted by
wire hundreds ofmiles to points where it is usefullyand
economically employed.*[9]*[140]The rst windmill for electricity
production was built inScotland in July 1887 by the Scottish
electrical engineerJames Blyth.*[141] Across the Atlantic, in
Cleveland,Ohio a larger and heavily engineered machine wasdesigned
and constructed in 188788 by Charles F.Brush,*[142] this was built
by his engineering companyat his home and operated from 1886 until
1900.*[143]The Brush wind turbine had a rotor 56 feet (17 m) in
di-ameter and was mounted on a 60-foot (18 m) tower. Al-though
large by today's standards, the machine was onlyrated at 12 kW; it
turned relatively slowly since it had 144blades. The connected
dynamo was used either to chargea bank of batteries or to operate
up to 100 incandescentlight bulbs, three arc lamps, and various
motors in Brush'slaboratory. The machine fell into disuse after
1900 whenelectricity became available from Cleveland's central
sta-tions, and was abandoned in 1908.*[144]
5 20th centuryVarious units of electricity and magnetism have
beenadopted and named by representatives of the electri-cal
engineering institutes of the world, which units andnames have been
conrmed and legalized by the govern-ments of the United States and
other countries. Thus thevolt, from the Italian Volta, has been
adopted as the prac-tical unit of electromotive force, the ohm,
from the enun-ciator of Ohm's law, as the practical unit of
resistance; theampere, after the eminent French scientist of that
name,as the practical unit of current strength, the henry as
thepractical unit of inductance, after Joseph Henry and
inrecognition of his early and important experimental workin mutual
induction.*[145]Dewar and John Ambrose Fleming predicted that
atabsolute zero, pure metals would become perfect elec-tromagnetic
conductors (though, later, Dewar altered hisopinion on the
disappearance of resistance believing thatthere would always be
some resistance). Walther Her-mann Nernst developed the third law
of thermodynam-ics and stated that absolute zero was unattainable.
Carlvon Linde and William Hampson, both commercial re-searchers,
nearly at the same time led for patents onthe Joule-Thomson eect.
Linde's patent was the cli-max of 20 years of systematic
investigation of establishedfacts, using a regenerative counterow
method. Hamp-son's design was also of a regenerative method. The
com-bined process became known as the Linde-Hampson liq-uefaction
process. Heike Kamerlingh Onnes purchaseda Linde machine for his
research. Zygmunt FlorentyWroblewski conducted research into
electrical proper-ties at low temperatures, though his research
ended earlydue to his accidental death. Around 1864, Karol
Ol-szewski andWroblewski predicted the electrical phenom-
ena of dropping resistance levels at ultra-cold tempera-tures.
Olszewski and Wroblewski documented evidenceof this in the 1880s. A
milestone was achieved on 10July 1908 when Onnes at the Leiden
University in Leidenproduced, for the rst time, liquied helium and
achievedsuperconductivity.In 1900, William Du Bois Duddell develops
the SingingArc and produced melodic sounds, from a low to a
high-tones, from this arc lamp.
5.1 Lorentz and PoincarMain articles: History of special
relativity and Lorentzether theoryBetween 1900 and 1910, many
scientists like Wilhelm
Hendrik Lorentz.
Wien, Max Abraham, Hermann Minkowski, or GustavMie believed that
all forces of nature are of electromag-netic origin (the
so-calledelectromagnetic world view). This was connected with the
electron theory developedbetween 1892 and 1904 by Hendrik Lorentz.
Lorentzintroduced a strict separation between matter (electrons)and
ether, whereby in his model the ether is completelymotionless, and
it won't be set in motion in the neighbor-hood of ponderable
matter. Contrary to other electronmodels before, the
electromagnetic eld of the ether ap-pears as a mediator between the
electrons, and changes inthis eld can propagate not faster than the
speed of light.In 1896, three years after submitting his thesis on
the
-
20 5 20TH CENTURY
Kerr eect, Pieter Zeeman disobeyed the direct ordersof his
supervisor and used laboratory equipment to mea-sure the splitting
of spectral lines by a strong magneticeld. Lorentz theoretically
explained the Zeeman eecton the basis of his theory, for which both
received theNobel Prize in Physics in 1902. A fundamental conceptof
Lorentz's theory in 1895 was the theorem of cor-responding
statesfor terms of order v/c. This theoremstates that a moving
observer (relative to the ether) makesthe same observations as a
resting observer. This theo-rem was extended for terms of all
orders by Lorentz in1904. Lorentz noticed, that it was necessary to
changethe space-time variables when changing frames and in-troduced
concepts like physical length contraction (1892)to explain the
MichelsonMorley experiment, and themathematical concept of local
time (1895) to explain theaberration of light and the Fizeau
experiment. That re-sulted in the formulation of the so-called
Lorentz trans-formation by Joseph Larmor (1897, 1900) and
Lorentz(1899, 1904).*[146]*[147]*[148] As Lorentz later noted(1921,
1928), he considered the time indicated by clocksresting in the
aether astruetime, while local time wasseen by him as a heuristic
working hypothesis and amath-ematical artice.*[149]*[150]
Therefore, Lorentz's theo-rem is seen by modern historians as being
a mathematicaltransformation from arealsystem resting in the
aetherinto actitioussystem in motion.*[146]*[147]*[148]
Henri Poincar.
Continuing the work of Lorentz, Henri Poincar be-tween 1895 and
1905 formulated on many occasions thePrinciple of Relativity and
tried to harmonize it withelectrodynamics. He declared simultaneity
only a con-
venient convention which depends on the speed of light,whereby
the constancy of the speed of light would be auseful postulate for
making the laws of nature as sim-ple as possible. In 1900 he
interpreted Lorentz's localtime as the result of clock
synchronization by light sig-nals, and introduced the
electromagnetic momentum bycomparing electromagnetic energy to what
he called actitious uidof mass m = E/c2 . And nally inJune and July
1905 he declared the relativity principlea general law of nature,
including gravitation. He cor-rected some mistakes of Lorentz and
proved the Lorentzcovariance of the electromagnetic equations.
Poincaralso suggested that there exist non-electrical forces to
sta-bilize the electron conguration and asserted that grav-itation
is a non-electrical force as well, contrary to theelectromagnetic
world view. However, historians pointedout that he still used the
notion of an ether and distin-guished betweenapparentandrealtime
and thereforedidn't invent special relativity in its modern
understand-ing.*[148]*[151]*[152]*[153]*[154]*[155]
5.2 Einstein's Annus MirabilisMain article: Annus Mirabilis
PapersIn 1905, while he was working in the patent oce,
Albert Einstein, 1905.
Albert Einstein had four papers published in the Annalender
Physik, the leading German physics journal. Theseare the papers
that history has come to call the AnnusMirabilis Papers:
His paper on the particulate nature of light put for-
-
5.3 Latter half of the 20th Century 21
ward the idea that certain experimental results, no-tably the
photoelectric eect, could be simply un-derstood from the postulate
that light interacts withmatter as discrete packets(quanta) of
energy,an idea that had been introduced by Max Planck in1900 as a
purely mathematical manipulation, andwhich seemed to contradict
contemporary wave the-ories of light (Einstein 1905a). This was the
onlywork of Einstein's that he himself called revolu-tionary.
His paper on Brownian motion explained the ran-dom movement of
very small objects as direct ev-idence of molecular action, thus
supporting theatomic theory. (Einstein 1905b)
His paper on the electrodynamics of moving bod-ies introduced
the radical theory of special relativ-ity, which showed that the
observed independenceof the speed of light on the observer's state
ofmotionrequired fundamental changes to the notion of
si-multaneity. Consequences of this include the time-space frame of
a moving body slowing down andcontracting (in the direction of
motion) relative tothe frame of the observer. This paper also
arguedthat the idea of a luminiferous aetherone of theleading
theoretical entities in physics at the timewas superuous. (Einstein
1905c)
In his paper on massenergy equivalence (previ-ously considered
to be distinct concepts), Einsteindeduced from his equations of
special relativity whatlater became the well-known expression: E =
mc2, suggesting that tiny amounts of mass could beconverted into
huge amounts of energy. (Einstein1905d)
All four papers are today recognized as
tremendousachievementsand hence 1905 is known as
Einstein's"Wonderful Year". At the time, however, they were
notnoticed by most physicists as being important, and manyof those
who did notice them rejected them outright.Some of this worksuch as
the theory of light quantaremained controversial for
years.*[156]*[157]
5.3 Latter half of the 20th Century
The rst formulation of a quantum theory describingradiation and
matter interaction is due to Paul AdrienMaurice Dirac, who, during
1920, was rst able tocompute the coecient of spontaneous emission
of anatom.*[158] Paul Dirac described the quantization of
theelectromagnetic eld as an ensemble of harmonic oscil-lators with
the introduction of the concept of creationand annihilation
operators of particles. In the followingyears, with contributions
from Wolfgang Pauli, EugeneWigner, Pascual Jordan, Werner
Heisenberg and an el-egant formulation of quantum electrodynamics
due to
Paul Adrien Maurice Dirac.
Enrico Fermi,*[159] physicists came to believe that,
inprinciple, it would be possible to perform any compu-tation for
any physical process involving photons andcharged particles.
However, further studies by FelixBloch with Arnold Nordsieck,*[160]
and Victor Weis-skopf,*[161] in 1937 and 1939, revealed that such
com-putations were reliable only at a rst order of
perturbationtheory, a problem already pointed out by Robert
Op-penheimer.*[162] At higher orders in the series inni-ties
emerged, making such computationsmeaningless andcasting serious
doubts on the internal consistency of thetheory itself. With no
solution for this problem known atthe time, it appeared that a
fundamental incompatibilityexisted between special relativity and
quantum mechan-ics.In December 1938, the German chemists OttoHahn
and Fritz Strassmann sent a manuscript toNaturwissenschaften
reporting they had detectedthe element barium after bombarding
uranium withneutrons;*[163] simultaneously, they communicatedthese
results to Lise Meitner. Meitner, and her nephewOtto Robert Frisch,
correctly interpreted these resultsas being nuclear ssion.*[164]
Frisch conrmed thisexperimentally on 13 January 1939.*[165] In
1944,Hahn received the Nobel Prize for Chemistry for thediscovery
of nuclear ssion. Some h