Laser voice communication
Introduction Laser, acronym for light amplification by
stimulated emission of radiation. Lasers are devices that amplify
light and produce coherent light beams, ranging from infrared to
ultraviolet. A light beam is coherent when its waves, or photons,
propagate in step, or in phase, with one another. Laser light,
therefore, can be made extremely intense, highly directional, and
very pure in colour (frequency). Laser devices now extend into the
X-ray frequency range. Masers are similar devices for
microwaves.PRINCIPLES OF OPERATION Lasers harness atoms to store
and emit light in a coherent fashion. The electrons in the atoms of
a laser medium are first pumped, or energized, to an excited state
by an energy source. They are then stimulated by external photons
to emit the stored energy in the form of photons, a process known
as stimulated emission. The photons emitted have a frequency
characteristic of the atoms and travel in step with the stimulating
photons. These photons in turn impinge on other excited atoms to
release more photons. Light amplification is achieved as the
photons move back and forth between two parallel mirrors,
triggering further stimulated emissions. At the same time the
intense, directional, and monochromatic laser light leaks through
one of the mirrors, which is only partially silvered.
Stimulated emission, the underlying process for laser action,
was first described theoretically by Albert Einstein in 1917. The
working principles of lasers were outlined by the American
physicists Arthur Schawlow and Charles Hard Townes in their 1958
patent application. The patent was granted, but was later
challenged by the American physicist and engineer Gordon Gould. In
1960 the American physicist Theodore Maiman observed the first
laser action in solid ruby. A year later a helium-neon gas laser
was built by the Iranian-born American physicist Ali Javan. Then in
1966 a liquid laser was constructed by the American physicist Peter
Sorokin. The United States Patent Office court in 1977 affirmed one
of Gould's claims over the working principles of the laser.TYPES OF
LASERS According to the laser medium used, lasers are generally
classified as solid state, gas, semiconductor, or
liquid.Solid-State Lasers The most common solid laser media are
rods of ruby crystals and neodymium-doped glasses and crystals. The
ends of the rod are fashioned into two parallel surfaces coated
with a highly reflecting non-metallic film. Solid-state lasers
offer the highest power output. They are usually operated in a
pulsed manner to generate a burst of light over a short time.
Bursts as short as 12 10-15 sec have been achieved, which are
useful in studying physical phenomena of very brief duration.
Pumping is achieved with light from xenon flash tubes, arc lamps,
or metal-vapour lamps. The frequency range has been expanded from
infrared (IR) to ultraviolet (UV) by multiplying the original laser
frequency with crystal-like potassium dihydrogen phosphate, which
are even shorter, and X-ray wavelengths, which are even shorter,
have been achieved by aiming laser beams at yttrium targets.Gas
Lasers Gas Lasers Intense red and green beams of argon and
neodymium laser light cross a room, their paths bending sharply as
they strike mirrors. Scientists use the unique properties of laser
light to perform experiments that were previously impossible. Not
all laser light is visible. Whether visible or not, the high
intensity of even stray light can be hazardous to the delicate
tissue of eyes. For this reason, anyone working with lasers should
have protective eyewear. The laser medium of a gas laser can be a
pure gas, a mixture of gases, or even metal vapour, and is usually
contained in a cylindrical glass or quartz tube. Two mirrors are
located outside the ends of the tube to form the laser cavity. Gas
lasers are pumped by ultraviolet light, electron beams, electric
current, or chemical reactions. The helium-neon laser is known for
its high frequency stability, colour purity, and minimal beam
spread. Carbon dioxide lasers are very efficient, and consequently
they are the most powerful continuous wave (CW)
lasers.Semiconductor Lasers The most compact of lasers, the
semiconductor laser usually consists of a junction between layers
of semiconductors with different electrical conducting properties.
The laser cavity is confined to the junction region by means of two
reflective boundaries. Gallium arsenide is the semiconductor most
commonly used. Semiconductor lasers are pumped by the direct
application of electrical current across the junction, and they can
be operated in the CW mode with better than 50 per cent efficiency.
A method that permits even more efficient use of energy has been
devised. It involves mounting tiny lasers vertically in such
circuits, to a density of more than a million per square
centimetre. Common uses for semiconductor lasers include CD players
and laser printers.Liquid Lasers The most common liquid laser media
are inorganic dyes contained in glass vessels. They are pumped by
intense flash lamps in a pulse mode or by a gas laser in the CW
mode. The frequency of a tunable dye laser can be adjusted with the
help of a prism inside the laser cavity. Free-Electron Lasers
Lasers using beams of electrons unattached to atoms and spiralling
around magnetic field lines to produce laser radiation were first
developed in 1977 and are now becoming important research
instruments. They are tunable, as are dye lasers, and in theory a
small number could cover the entire spectrum from infrared to
X-rays. Free-electron lasers should also become capable of
generating very high-power radiation, which is currently too
expensive to produce.
Ruby laser being used in a Q-switch, a special switching device
that produces giant output pulse
Communication
Communication between a large number of information sources has
been accomplished through the organization of the sources into
networks. Networking has made it possible to concentrate
communications traffic in localized areas, with interconnections
provided between the various points of concentration to effect
efficient and economical distribution of transmission signals.
Telephone and telegraph networks (e.g., Telex) have long been in
operation. More recently, cable television and telemetry networks
(such as those used in the remote control of automobile traffic and
widely distributed industrial operations) have become prominent.
Rapid advances in computer technology also have led to a dramatic
growth of data-retrieval and exchange networks. These networks are
composed of individual computers and intelligent"peripheral
equipment (e.g., automatic teller machines and point-of-sale
terminals) that are interconnected by telephone lines, microwave
relays, and other high-speed communications links. Computer
networking has developed on all levels, from local to
international. Computer-to-computer communication has also become
commonplace for owners of home computers. Subscribers to special
information services can interact with a host computer to access
educational and entertainment materials as well as news and
stock-market reports. Laser light can travel a large distance in
outer space with little reduction in signal strength. Lasers are
therefore ideal for space communications. Because of its high
frequency, laser light can carry, for example, 1,000 times as many
television channels as are now carried by microwaves. Low-loss
optical fibres have been developed to transmit laser light for
earthbound communication in telephone and computer systems. Laser
techniques have also been used for high-density information
recording. For instance, laser light simplifies the recording of a
hologram, from which a three-dimensional image can be reconstructed
with a laser beam.LASER SAFETY Because the eye focuses laser light
as it does other light, the chief danger in working with lasers is
eye damage. Therefore, laser light should not be viewed, whether it
is direct or reflected. Lasers should be used only by trained
personnel wearing protective goggles.
Laser
Laser is a device that produces a very narrow, powerful beam of
light. Some beams are thin enough to drill 200 holes on a spot as
tiny as the head of a pin. The ability to focus laser light so
precisely makes it extremely powerful. For example, some beams can
pierce a diamond, the hardest natural substance. Others can trigger
a small nuclear reaction. A laser beam also can be transmitted over
long distances with no loss of power. Some beams have reached the
moon.
The special qualities of laser light make it ideal for a variety
of applications. Some types of lasers, for example, are used to
play music, read price codes, cut and weld metal, and transmit
information. Lasers can also guide a missile to a target, repair
damaged eyes, and produce spectacular displays of light. Still
other lasers are used to align walls and ceilings in a building or
to print documents. Some lasers even can detect the slightest
movement of a continent.
Lasers vary greatly in size. One is almost as long as a football
field. Another type is as small as a grain of salt.
A typical laser has three main parts. These parts are (1) an
energy source, (2) a substance called an active medium, and (3) a
structure enclosing the active medium known as an optical cavity.
The energy source supplies an electric current, light, or other
form of energy. The atoms of the active medium can absorb the
energy, store it for a while, and release the energy as light. Some
of this light triggers other atoms to release their energy. More
light is added to the triggering light. Mirrors at the ends of the
optical cavity reflect the light back into the active medium. The
reflected light causes more atoms to give off light. The light
grows stronger, and part of it emerges from the laser as a narrow
beam. Some beams are visible. Others consist of invisible forms of
radiation.
There are four main kinds of lasers. They are solid-state
lasers, semiconductor lasers, gas lasers, and dye lasers.
In 1960, the American physicist Theodore Maiman built the first
laser. At first, lasers had few uses, and scientists often thought
of them as "a solution looking for a problem." Today, however,
lasers are among the most versatile and important tools in modern
life.
How lasers are used
Lasers can do a number of incredible things. Their special
qualities make them particularly useful in recording, storing, and
transmitting many kinds of information. Lasers also are valuable in
such activities as scanning, heating, measuring, and guiding. As a
result of their wide use, lasers can be found in equipment used in
homes, factories, offices, hospitals, and libraries.
Recording, storing, and transmitting information. The most
common uses of lasers include the recording of music, films,
computer data, and other material on special discs. Bursts of laser
light record such material on the discs in patterns of tiny pits.
The discs with recorded music and computer data are called compact
discs (CD's). A laser beam's tight focus allows much more
information to be stored on a CD than on a gramophone record,
making CD's good for holding data as well as music. Some CD's even
can hold an entire encyclopedia. A disc used for storing data is
usually called a CD-ROM (Compact Disc Read-Only Memory). Such discs
store databases (large files of information held in computers) and
are used widely by businesses, libraries, and government
agencies.
Lasers can also read and play back the information recorded on
discs. In a CD player, a laser beam reflects off the pattern of
pits as the compact disc spins. Other devices in the player change
the reflections into electrical signals and decode them as music.
More lasers are used in CD players than in any other product.
Lasers are used to record films on large platters called
videodiscs. In addition, laser beams can produce three-dimensional
images in a photographic process called holography. The images,
recorded on a photographic plate, are known as holograms. They
appear in advertising displays, artwork, and jewellery, and some
are placed on credit cards to prevent counterfeiting.
One of the laser's greatest uses is in the field of fibre-optics
communication. This technology changes electrical signals of
telephone calls and television pictures into pulses (bursts) of
laser light. Strands of glass called optical fibres conduct the
light. An optical fiber is about as thin as a human hair. But one
fibre can carry as much information as several thousand copper
telephone wires. Laser light is ideal for this technology because
it can be focused precisely and because all its energy can be
introduced into the fibre. Fibre-optic transmission of laser light
allows enormous amounts of telephone, TV, and other data to be
communicated relatively cheaply.
Scanning involves the movement of a laser beam across a surface.
Scanning beams are often used to read information. Many people have
become familiar with laser scanners used at supermarket checkout
counters. What looks like a line of light is actually a rapidly
moving laser beam scanning a bar code. A bar code consists of a
pattern of lines and spaces on packages that identifies the
product. The scanner reads the pattern and sends the information to
a computer in the supermarket. The computer identifies the item's
price and sends the information to the register. Many other kinds
of stores use bar code scanners. In addition, such scanners keep
track of books in libraries, sort mail in post offices, and read
account numbers on cheques in banks. Laser printers use a scanning
laser beam to produce copies of documents. Other scanners make
printing plates for newspapers.
In entertainment, laser light shows are created with scanning
laser beams. These beams can "draw" spectacular patterns of red,
yellow, green, and blue light on buildings or other outdoor
surfaces. The beams move so rapidly they produce what looks like a
stationary picture. Laser scanners also produce colourful visual
effects that create excitement at rock concerts.
Heating. A laser beam's highly focused energy can produce a
great amount of heat. Industrial lasers, for example, produce beams
of thousands of watts of power. They cut and weld metals, drill
holes, and strengthen materials by heating them. Industrial lasers
also cut ceramics, cloth, and plastics. In medicine, the heating
power of lasers is often used in eye surgery. Highly focused beams
can close off broken blood vessels on the retina, a tissue in the
back of the eyeball. Lasers also can reattach a loose retina. Laser
beams pass through the cornea (front surface of the eye) but cause
no pain or damage because the cornea is transparent and does not
absorb light.
Doctors also use lasers to treat skin disorders, remove
birthmarks, and shatter gallstones. Laser beams can replace the
standard surgical knife, or scalpel, in some operations. The use of
lasers permits extraordinary control and precision in cutting
tissue and sealing off cuts. Thus, lasers reduce bleeding and
damage to nearby healthy tissues. In nuclear energy research,
scientists use lasers to produce controlled, miniature hydrogen
bomb explosions. They focus many powerful laser beams onto a pellet
of frozen forms of hydrogen. The intense beams compress (pack down)
the pellet and heat it to millions of degrees. These actions cause
the pellet's atoms to fuse (unite) and release energy. This
process, called nuclear fusion, may produce enough energy to solve
the world's energy problems. Lasers have produced the tremendous
heat needed to create fusion but have not yet produced usable
amounts of energy.
Measuring. People also use lasers to measure distance. An
object's distance can be determined by measuring the time a pulse
of laser light takes to reach and reflect back from the object. In
1969 and 1971, United States astronauts placed mirrored devices
called laser reflectors on the moon. Using a high-powered laser,
scientists measured the distance between the earth and the
moon--more than 383,000 kilometres--to within 5 centimetres. They
made the measurement by shining laser light from a telescope on the
earth to the reflectors on the moon.
Laser beams directed over long distances also can detect small
movements of the ground. Such measurements help geologists involved
in earthquake warning systems. Laser devices used to measure
shorter distances are called range finders. Surveyors use the
devices to get information needed to make maps. Military personnel
use laser range finders to calculate the distance to an enemy
target.
Guiding. A laser's strong, straight beam makes it a valuable
tool for guidance. For example, construction workers use laser
beams as "weightless strings" to align the walls and ceilings of a
building and to lay straight sewer and water pipes. Instruments
called laser gyroscopes use laser beams to detect changes in
direction. These devices help ships, aeroplanes, and guided
missiles stay on course. Another military use of lasers is in a
guidance device called a target designator. A person using the
device aims a laser beam at an enemy target. Missiles, artillery
shells, and bombs equipped with laser beam detectors seek the
reflected beam and adjust their flight to hit the spot where the
beam is aimed.
How a laser works
Parts of a laser. A typical laser has three main parts. These
parts are an active medium, an energy source, and an optical
cavity. An active medium is a material that can be made to create
laser light. Gases, liquids, or solid materials can be used. An
energy source is any type of device that supplies energy to the
active medium in a process called pumping. Lasers often use
electricity, another laser, or a flash lamp as an energy pump. A
flash lamp produces a bright flash of light, just as a camera flash
does. An optical cavity, also called a resonator, is a structure
that encloses the active medium. A typical cavity has a mirror at
each end. One mirror has a fully reflecting surface, and the other
one has a partly reflecting surface. The laser beam exits the laser
through the mirror with the partly reflecting surface.
The nature of atoms. Laser light results from changes in the
amount of energy stored by the atoms in an active medium. The atoms
of a substance normally exist in a state of lowest energy, called a
ground state. Atoms also can exist in higher energy states, called
excited states. Atoms can change from a ground state to an excited
state by absorbing various forms of energy. This process is called
absorption. In many lasers, atoms absorb packets of light energy
called photons. In most instances, the excited atom can hold the
extra energy for only a fraction of a second before the atom
releases its energy as another photon and falls back to its ground
state. This process is called spontaneous emission. Some atoms have
excited states that can store energy for a relatively long time.
These long-lived states can last as long as 1/1,000 of a
second--much longer than the duration of most excited states. When
a photon of just the right amount of energy shines on an atom in a
long-lived excited state, it can stimulate the atom to emit (give
off) an identical photon. This second photon has an equal amount of
energy and moves in the same direction as the original photon. This
process is called stimulated emission. Producing laser light.
Stimulated emission is the central process of a laser. One
photon--the stimulating photon--produces another photon. It doubles
the amount of light energy present, a process called amplification.
The word laser comes from the first letters of the words that
describe the key processes in the creation of laser light. These
words are light amplification by stimulated emission of radiation.
Stimulated emission only occurs if there are atoms in the excited
state. However, atoms in the ground state generally greatly
outnumber those in excited states. For amplification to take place,
more atoms of a substance must exist in excited states than in
ground states. This condition is called a population inversion. In
a laser, the energy source helps create a population inversion by
pumping energy into the active medium. This energy places atoms in
long-lived excited states and enables stimulated emission to occur.
The mirrors in the optical cavity reflect the photons back and
forth in the active medium.
Each interaction of a photon and an excited atom produces a
chain reaction of stimulated emissions. This chain reaction causes
the number of stimulated emissions to increase rapidly and produce
a flood of light. Part of this intense light exits through the
partly reflecting mirror as a strong beam. Characteristics of laser
light. Laser light differs from ordinary light in two major ways.
(1) It has low divergence (spreading). (2) It is monochromatic
(single-coloured). Light with these two characteristics is known as
coherent light. Most sources of light diverge rapidly. Light from a
flashlight, for example, fans out quickly and fades after a short
distance. But laser light travels in an extremely narrow beam. It
spreads little, even over long distances. For example, a typical
laser beam expands to a diameter of only 1 metre after travelling
1,000 metres, or only about 260 centimetres per kilometre.
Light consists of electromagnetic waves, and the colour of light
is determined by its wavelength (distance from one peak of a wave
to the next). Ordinary light consists of waves of many
wavelengths--and colours. When all these waves are seen together at
the same time, their colours appear white--like those from a light
bulb. But light produced by most lasers consists of waves with a
very narrow range of wavelengths. Because this range is so narrow,
laser light appears to consist of a single colour. Some lasers can
produce beams with several different colours, but each colour band
will be narrow. Some lasers produce an invisible beam. These beams
consist of such forms of radiation as ultraviolet or infrared
rays.
Laser light is highly organized, or coherent. The waves of a
laser beam move in phase--that is, all the peaks move in step with
one another. These waves travel in a narrow path and move in one
direction. Thus, coherent light is like a line of marchers in a
parade moving with the same strides in the same direction. The
waves of ordinary light, on the other hand, spread rapidly and
travel in different directions. Ordinary light is known as
incoherent light. Incoherent light acts much like the way people
usually travel along a street--with different strides and in many
directions. A laser beam's coherence allows it to travel long
distances without losing its intensity.
Kinds of lasers
Most lasers can produce light either in a continuous beam or in
pulses. The lasers that generate pulses, which are called pulsed
lasers, supply all their energy in only a fraction of a second. As
a result, they generally produce much greater peak power than
lasers that produce a continuous beam, which are called
continuous-wave lasers. Most continuous-wave lasers range in power
from less than 1/1,000 of a watt to more than 10,000 watts. But
some pulsed lasers can produce beams of several trillion watts for
a billionth of a second.
There are four main types of lasers. These types are (1)
solid-state lasers, (2) semiconductor lasers, (3) gas lasers, and
(4) dye lasers.
Solid-state lasers use a rod made of a solid material as the
active medium. Substances made of crystals or glass are widely
used. The most common crystal laser contains a small amount of the
element neodymium (chemical symbol Nd) in an yttrium aluminium
garnet (YAG) crystal. It is called an Nd:YAG laser. In some lasers,
the neodymium is dissolved in glass. Flash lamps are generally used
to pump the active media of solid-state lasers. The world's largest
laser is an Nd: glass laser at Lawrence Livermore National
Laboratory in Livermore, California, U.S.A. This laser, called
Nova, is about as long as a football field. It produces laser light
in pulses and is used for nuclear energy research. Its light is
split into 10 beams, which are amplified to focus more than 100
trillion watts of power on a target for a billionth of a second.
The world's most powerful laser, the Petawatt, uses one of Nova's
beam lines. The Petawatt has produced more than 1 quadrillion
(1,000 trillion) watts in a pulse lasting less than 500
quadrillionths of 1 second. Nd:YAG and Nd:glass lasers are used
widely in industry to drill and weld metals. They are also found in
range finders and target designators. Semiconductor lasers, also
called diode lasers, use semiconductors, which are materials that
conduct electricity but do not conduct it as well as copper, iron,
or other true conductors. Semiconductors used in lasers include
compounds of metals such as gallium, indium, and arsenic. The
semiconductor in a laser consists of two layers that differ in
their electric properties. The junction between the layers serves
as the active medium. When current flows across the junction, a
population inversion is produced. Flat ends of the semiconductor
materials serve as mirrors and reflect the photons. Stimulated
emission occurs in the junction region. Semiconductor lasers are
the smallest type of laser. One kind is as tiny as a grain of salt.
Another type is even smaller and can be seen only with a
microscope. Semiconductor lasers are the most commonly used type of
laser because they are smaller and lighter and use less power than
the other kinds. Their size makes them ideal for use in CD and
videodisc players and for fibre-optic communications.
Gas lasers use a gas or mixture of gases in a tube as the active
medium. The most common active media in gas lasers include carbon
dioxide, argon, krypton, and a mixture of helium and neon. The
atoms in gas lasers are excited by an electric current in the same
way that neon signs are made to light. Gas lasers are commonly used
in communications, eye surgery, entertainment, holography,
printing, and scanning. Many gas lasers produce infrared beams. The
most important one is the carbon dioxide laser. It is among the
most efficient and powerful lasers.
Carbon dioxide lasers convert 5 to 30 per cent of the energy
from their energy source into laser light. Many other lasers
convert only about 1 per cent of the energy they get. Carbon
dioxide lasers can produce beams ranging from less than 1 watt to
more than 1 million watts. They are often used to weld and cut
metals. They also are used as laser scalpels and in range finders.
Dye lasers use a dye as the active medium. Many kinds of dyes can
be used. The dye is dissolved in a liquid, often alcohol. A second
laser is generally used to pump the atoms of the dye. The most
important property of dye lasers is that they are tunable--that is,
a single laser can be adjusted to produce monochromatic beams over
a range of wavelengths, or colours. Tunable lasers are valuable to
researchers who investigate how materials absorb different colours
of light.
LDR A photoresistor is an electronic component whose resistance
decreases with increasing incident light intensity. It can also be
referred to as a light-dependent resistor (LDR), or
photoconductor.A photoresistor is made of a high-resistance
semiconductor. If light falling on the device is of high enough
frequency, photons absorbed by the semiconductor give bound
electrons enough energy to jump into the conduction band. The
resulting free electron (and its hole partner) conduct electricity,
thereby lowering resistance.A photoelectric device can be either
intrinsic or extrinsic. An intrinsic semiconductor has its own
charge carriers and is not an efficient semiconductor, eg. silicon.
In intrinsic devices, the only available electrons are in the
valence band, and hence the photon must have enough energy to
excite the electron across the entire bandgap. Extrinsic devices
have impurities added, which have a ground state energy closer to
the conduction band since the electrons don't have as far to jump,
lower energy photons (i.e. longer wavelengths and lower
frequencies) are sufficient to trigger the device. If a sample of
silicon has some of its atoms replaced by phosphorus
atoms(impurities), there will be extra electrons available for
conduction. This is an example of an extrinsic semiconductor.
Some photosensitive elements like cedmium sulphide, cedmium
selenide etc. emites electrons when receive light. The resistance
of these elements in the dark is in mega ohm but remains in few ohm
when they get light. Light dependent resistance are made by stiking
photo sensitive strip on the base of the insulating material. In
this the two strips of cediuium sulphide are seprated by a small
gap, this appears as if two combs are joined together. It is shown
in the fig. 25
L. D. R. In the above figure black lines are layer of photo
sensitive elements and white colored element is the ceremic base.
These types of resistances are used in some television recieves for
automatic brightness and contrast control. Besides this, these are
mainly used in the automatic flash system of cameras. In this type
of cameras there is small hole near the lens through which lights
comes on the LDR. Whenever the light is dim, the value of LDR
became high, as a result capacitor chargers due to high value load
resistance in the charging circuit thus flash does it work. But
when there is enough sun light than LDR gets light so the value
load resistance become low and charging circuit does not works so
flash does not works because capacitor does not charges. Besides
this, LDR is used in many other devices.
BATTERY
Battery is a device that produces electricity by means of
chemical action. A battery consists of one or more units called
electric cells. Each cell has all the chemicals and parts needed to
produce an electric current. The word battery actually means a
group of connected cells. However, the term is generally used to
refer to single cells, such as those used in torches and electric
toys.
Batteries serve as a convenient source of electricity. They
power such portable equipment as radios, tape recorders, and
television sets. In a car, a battery provides power to start the
engine. Batteries also supply electricity in spacecraft and
submarines. During power failures, batteries provide an emergency
supply of electricity for telephones, fire alarms, and hospitals
and other essential buildings.
Kinds of batteries
Manufacturers produce a wide variety of batteries, which may be
classified according to their basic design. The design of a battery
determines the amount of electricity provided. Some batteries,
called primary batteries, stop working and must be discarded after
one of their chemicals has been used up. Other batteries can be
recharged and used again after they have discharged their
electrical energy. They are called secondary, or storage,
batteries.
Batteries may also be classified according to the general makeup
of their electrolyte, the chemical substance that conducts the
electric current inside a cell. Many primary batteries have a
jellylike or pastelike electrolyte. Batteries that contain such
non-spillable material are known as dry cells. A few types of
primary batteries, called wet cells, contain liquid chemicals. Most
secondary batteries have a liquid electrolyte. Batteries are
manufactured in a wide range of sizes. For example, the tiny
batteries used in electric watches weigh only about 1.4 grams. The
huge batteries that power submarines weigh up to 1 metric ton.
However, manufacturers produce most batteries in certain standard
sizes. Therefore, batteries made by different manufacturers can be
used in the same device. Batteries also differ in voltage. A
primary cell of the type used in a torch is a 1.5-volt battery.
Most secondary batteries for cars are 12-volt batteries consisting
of six 2-volt cells connected in a series.
How dry primary batteries work
Dry primary batteries are the most widely used type of primary
cell. Such batteries differ in various ways, but all have certain
basic parts. Every dry primary battery has two structures called
electrodes. Each electrode consists of a different kind of
chemically active material. An electrolyte between the electrodes
causes one of them, called the anode, to become negatively charged
and the other, called the cathode, to become positively charged.
The electrolyte helps promote the chemical reactions that occur at
the electrodes.
There are three major types of dry primary batteries. They are
(1) carbon-zinc cells, (2) alkaline cells, and (3) mercury cells.
Carbon-zinc cells are the general-purpose batteries used in
torches, photoflash units, and toys. These cells, also called
Leclanche dry cells, are contained within a zinc cannister. The
cannister serves both as a container for the parts of the cell and
as the anode. A carbon rod in the centre of the cell functions as
the cathode current-collector. But the actual cathode material is a
mixture of manganese dioxide and carbon powder packed around the
rod. The electrolyte is a paste composed of ammonium chloride, zinc
chloride, and water.
The anode and the cathode are separated by a sheet of porous
material, such as paper or cardboard, soaked with the electrolyte.
This thin layer, called a separator, prevents the electrode
materials from mixing together and reacting when a battery is not
being used. Such action could cause the zinc anode to wear away
prematurely and reduce the life of the battery. The chemical
process that produces electricity begins when the atoms of zinc
(Zn) at the surface of the anode oxidize. A zinc atom oxidizes when
it gives up both its electrons. It then becomes an ion (an
electrically charged atom) with a positive charge. The zinc ions
move away from the anode. As they do so, they leave their electrons
behind on its surface. The anode thus gains an excess of electrons
and becomes more negatively charged than the cathode. If a cell is
connected to an external circuit, the zinc anode's excess electrons
flow through the circuit to the carbon rod. The movement of
electrons forms an electric current. After the electrons enter the
cell through the rod, they combine with molecules of manganese
dioxide and molecules of water. As these substances are reduced
(gain electrons) and react with one another, they produce manganese
oxide and negative hydroxide ions. This reaction makes up the
second half of the cell's discharge process. It is accompanied by a
secondary reaction. In the secondary reaction, the negative
hydroxide ions combine with positive ammonium ions that form when
ammonium chloride is dissolved in water. The secondary reaction
produces molecules of ammonia and molecules of water.
The various chemical reactions by which a carbon-zinc cell
produces electricity continue until the manganese dioxide wears
away. After this cathode material has been "used up," the cell can
no longer provide useful energy and is dead.
Dead cells should be removed immediately. Even after a cell
stops working, its electrolyte continues to eat away at the
container and may puncture it. If the electrolyte leaks out, it can
damage the equipment.
A carbon-zinc cell, like most primary batteries, cannot be
recharged efficiently. But a device called a battery charger may
extend the life of a cell for a short time. It partially restores
the cell's ability to produce electricity. A battery charger
functions by passing a current through the cell in a direction
opposite to that of the flow of electricity during discharge.
Transistor
Transistor, any of various electronic devices used as amplifiers
or oscillators in communications, control, and computer systems.
Since its advent in 1948 the transistor has largely replaced
thermionic vacuum tubes.
Circuit Board and Transistors: - A close-up of a smoke detectors
circuit board reveals its components, which include transistors,
resistors, capacitors, diodes, and inductors. Rounded containers
house the transistors that make the circuit work. Transistors are
capable of serving many functions, such as amplifying and
switching. Each transistor consists of a small piece of
semiconducting material, such as silicon, that has been doped, or
treated with impurity atoms, to create n-type and p-type regions.
Invented in 1948, transistors are a fundamental component in nearly
all modern electronic devices. Capable of performing many functions
of the vacuum tube in electronic circuits, the transistor is a
solid-state device consisting of a tiny piece of semiconductor,
usually germanium or silicon, to which three or more electrical
connections are made. The basic components of the transistor are
comparable to those of a triode vacuum tube and include the
emitter, which corresponds to the heated cathode of the triode tube
as the source of electrons.
Bipolar Junction Transistors:- The bipolar junction transistor
consists of three layers of highly purified silicon (or germanium)
to which small amounts of boron (p-type) or phosphorus (n-type)
have been added. The boundary between each layer forms a junction,
which only allows current to flow from p to n. Connections to each
layer are made by evaporating aluminium onto the surface; the
silicon dioxide coating protects the nonmetallized areas. A small
current through the base-emitter junction causes a current 10 to
1,000 times larger to flow between the collector and emitter. (The
arrows show a positive current; the names of layers should not be
taken literally.) The many uses of the junction transistor, from
sensitive electronic detectors to powerful hi-fi amplifiers, all
depend on this current amplification. The transistor was developed
at Bell Laboratories by the American physicists Walter Houser
Brattain, John Bardeen, and William Bradford Shockley. For this
achievement and their pioneering research on semiconductors, the
three shared the 1956 Nobel Prize for Physics. Shockley is noted as
the initiator and director of the research programme in
semiconducting materials that led to the discovery of this group of
devices; his associates, Brattain and Bardeen, are credited with
the invention of an important type of transistor.
ATOMIC STRUCTURE OF SEMICONDUCTORS
The electrical properties of a semiconducting material are
determined by its atomic structure. In a crystal of pure germanium
or silicon, the atoms are bound together in a periodic arrangement
forming a perfectly regular diamond-cubic lattice. Each atom in the
crystal has four valence electrons, each of which interacts with
the electron of a neighbouring atom to form a bond. Because the
electrons are not free to move, the pure crystalline material acts,
at low temperatures, as an insulator.FUNCTION OF IMPURITIES
Germanium or silicon crystals containing small amounts of
certain impurities can conduct electricity even at low
temperatures. Such impurities function in the crystal in one of two
ways. An impurity element such as phosphorus, antimony, or arsenic
is called a donor impurity because it contributes excess electrons.
This group of elements has five valence electrons, only four of
which enter into bonding with the germanium or silicon atoms. Thus,
when an electronic field is applied, the remaining electrons in
donor impurities are free to move through the crystalline material.
In contrast, impurity elements such as gallium and indium have only
three valence electrons, lacking one to complete the
interatomic-bond structure within the crystal. Such impurities are
known as acceptor impurities because these elements accept
electrons from neighbouring atoms to satisfy the deficiency in
valence-bond structure. The resultant deficiencies, or so-called
holes, in the structure of neighbouring atoms, in turn, are filled
by other electrons and so on. These holes behave as positive
charges, appearing to move under an applied voltage in a direction
opposite to that of the electrons.N-TYPE AND P-TYPE
SEMICONDUCTORS
Figure 1: N-P Junction An n-p junction (also known as a diode)
will allow current to flow only in one direction. The electrons
from the n-type material can pass to the right through the p-type
material, but the lack of excess electrons in the p-type material
will prevent any flow of electrons to the left. Note that the
current is defined as flowing in a direction that is opposite to
the direction of flow of the electrons. A germanium or silicon
crystal, containing donor-impurity atoms, is called a negative, or
n-type, semiconductor to indicate the presence of excess negatively
charged electrons. The use of an acceptor impurity produces a
positive, or p-type, semiconductor, so called because of the
presence of positively charged holes. A single crystal containing
both n-type and p-type regions may be prepared by introducing the
donor and acceptor impurities into molten germanium or silicon in a
crucible at different stages of crystal formation. The resultant
crystal has two distinct regions of n-type and p-type material, and
the boundary joining the two areas is known as an n-p junction.
Such a junction may also be produced by placing a piece of
donor-impurity material against the surface of a p-type crystal or
a piece of acceptor-impurity material against an n-type crystal and
applying heat to diffuse the impurity atoms through the outer
layer. When an external voltage is applied (figure 1), the n-p
junction acts as a rectifier, permitting current to flow in only
one direction. If the p-type region is connected to the positive
terminal of a battery and the n-type to the negative terminal, a
large current flows through the material across the junction. If
the battery is connected in the opposite manner, current does not
flow.
Circuit Board and Transistors
A close-up of a smoke detectors circuit board reveals its
components, which include transistors, resistors, capacitors,
diodes, and inductors. Rounded containers house the transistors
that make the circuit work. Transistors are capable of serving many
functions, such as amplifying and switching. Each transistor
consists of a small piece of semiconducting material, such as
silicon, that has been doped, or treated with impurity atoms, to
create n-type and p-type regions. Invented in 1948, transistors are
a fundamental component in nearly all modern electronic
devices.
Bipolar Junction Transistors The bipolar junction transistor
consists of three layers of highly purified silicon (or germanium)
to which small amounts of boron (p-type) or phosphorus (n-type)
have been added. The boundary between each layer forms a junction,
which only allows current to flow from p to n. Connections to each
layer are made by evaporating aluminium onto the surface; the
silicon dioxide coating protects the nonmetallized areas. A small
current through the base-emitter junction causes a current 10 to
1,000 times larger to flow between the collector and emitter. (The
arrows show a positive current; the names of layers should not be
taken literally.) The many uses of the junction transistor, from
sensitive electronic detectors to powerful hi-fi amplifiers, all
depend on this current amplification.
Figure 1: N-P Junction
An n-p junction (also known as a diode) will allow current to
flow only in one direction. The electrons from the n-type material
can pass to the right through the p-type material, but the lack of
excess electrons in the p-type material will prevent any flow of
electrons to the left. Note that the current is defined as flowing
in a direction that is opposite to the direction of flow of the
electrons.
Figure 2: N-P-N Transistor Amplifier The voltage from a source
is applied to the base of the transistor (labelled P). Small
changes in this applied voltage across R1 (input) result in large
changes in the voltage across the resistor labelled R2 (output).
One possible application of this circuit would be to amplify
sounds. In this case the input would be a microphone and the
resistor R2 would be a speaker. Hi-fi amplifiers have many more
transistors, both to increase the power output and to reduce the
distortion that occurs in simple circuits like this.