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.