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Laser beams are used for obtaining accurate alignment of parts in engineering projects such as the construction of buildings and the laying of pipelines. Precise measurement of long distances can be made by determining the time it takes for a pulse of light from a laser to travel to a distant object and return. Using this method, scientists have measured precisely the distance between the earth and the moon by directing laser beams from earth onto reflectors left on the moon by Apollo astronauts.

The intense light of powerful lasers can cut through or heat many materials more quickly and easily than can conventional tools. In industry, laser beams are used for a variety of manufacturing operations, from cutting textiles to drilling holes through hard metals. A laser can produce intense heating over a small area, making it ideal for certain types of welding.

Lasers are used for delicate operations on the eye, brain, and other organs where conventional operating methods are often difficult. In surgery to repair detached retinas, for example, the laser beam can be passed through the lens of the eye and, in effect, weld breaks in the retina; the lens and other transparent parts of the eye are unaffected by the beam. When lasers are used as surgical knives, the heat from the laser beam immediately seals off severed blood vessels in the tissue being cut, thus minimizing bleeding.

Tiny lasers that can be turned on and off millions of times a second are used to transmit telephone messages and other information as flashes of light through optical fibers. Lasers are also used in certain electronic devices to read specially encoded information. For example, some supermarket optical scanners use a laser to read the Universal Product Code on grocery items at the checkout counter, and compact disc players use a laser to read data recorded on a small plastic disc.

Laser-guided missiles have proven highly accurate; the target is illuminated by the laser beam and the missile's guidance system uses the beam's reflection to home in on the target. Experimental lasers that can disable missiles at a distance in the air have been successfully tested.

Lasers are used by chemists to analyze the composition of chemical substances and to study chemical reactions. In the field of nuclear energy, some scientists think very powerful lasers are the key to making a successful fusion reactor. The lasers would provide the tremendous heat needed to start the fusion process.

Another important application of lasers is in holography, a method of producing three-dimensional images.

Basic Principles

(Note—It is suggested that the article Radiation, introduction and subtitle Electromagnetic Radiation be read before reading the technical sections of this article.)

The laser makes use of some of the properties of the atom to generate or amplify light waves. Atoms absorb and emit energy in the form of photons (small bundles or particles of energy). An atom that has absorbed a photon is said to be excited (raised in energy). In returning to its normal, or ground, state, the atom gives up one or more photons. Usually, excited atoms return to the ground state in a spontaneous, random manner. However, an excited atom can be stimulated to give up a photon if the atom is hit by an outside photon having exactly the same energy as the photon the atom would have given up spontaneously. The outside photon and the photon given up move away from the atom and travel in the same direction and in coherent waves.

Any given atom can absorb and emit only photons with particular amounts of energy (particular wavelengths). Which wavelengths can be absorbed and emitted depends on which kind of atom is being used.

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Usually most of the atoms in a substance are in the ground state. In a laser, most of the atoms are excited at one time, and are then made to emit their photons in an orderly way. When the photons are emitted, an intense, directional, and coherent beam is generated by the laser. The particular properties of the beam generated depend on the material used for the laser. A solid-state laser uses either a crystal or glass; other kinds of lasers use gases, liquids, or semiconductors.

Solid-state Laser

A characteristic type of solid-state laser is the ruby laser. It contains a rod-shaped crystal of synthetic ruby. This material, like natural ruby gemstones, is composed of aluminum oxide with trace amounts of chromium. The ends of the rod are cut parallel to each other and perpendicular to the sides. One end is heavily silveredthat is, coated with silver or other reflective materialso that it reflects almost all the light that strikes it. The other end is lightly silvered so that some of the light will be reflected and the rest will pass through.

A lamp that can produce an intense flash of white light surrounds the ruby crystal. When the lamp flashes, most of the chromium atoms in the crystal absorb photons. Within a fraction of a second, these excited atoms begin returning to the ground state, and, in the process, emit photons. When these photons strike chromium atoms that are still excited, they stimulate them to emit yet other photons. As this process continues, beams of coherent light are formed. The beam that is parallel to the sides of the ruby is reflected back and forth between the silvered ends until sufficient photons have joined it to make it powerful enough to escape through the lightly silvered end. (The beams that are not parallel to the sides escape from the ruby before they have a chance to build up much intensity.)

The ruby laser emits a red light. Other types of solid-state lasers can be used to generate light of other colors or to generate infrared radiation.

Gas Laser

A gas laser contains one or more gases sealed in a glass tube. Two mirrors. one heavily silvered and the other lightly silvered, lie at either end of the tube. The most common type of gas laser uses a mixture of helium and neon. Other gases used include carbon dioxide and argon. In the helium-neon laser, energy is provided by an electrical discharge that excites the helium atoms. When the excited helium atoms collide with the neon atoms, the energy is transferred from the helium to the neon. The neon atoms emit photons that form a laser beam in essentially the same way as in the ruby laser. Helium-neon lasers can produce beams of red light, green light, or infrared radiation, depending on the mirrors used.

Liquid Laser

The typical liquid laser uses a fluorescent dye in a glass tube. As in the gas laser, two mirrors, one heavily silvered and the other lightly silvered, lie at either end of the tube. Energy to excite the molecules of the dye is provided either by a flash lamp or by an ultraviolet laser. Liquid lasers can be made to produce extremely brief pulses of light; some have produced pulses lasting less than a trillionth of a second. Dye lasers are used to produce beams of visible light of almost any color.

Semiconductor Laser

A semiconductor laser is essentially a kind of electronic device called a junction diode. It is made of a semiconductor, typically gallium arsenide, that has been treated to form two types of materialsan n-type material, which has an excess of electrons, and a p-type material, which has a deficiency of electrons. The p-type material contains positively charged vacancies called holes.

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When a voltage is applied across the diode, excess electrons of the n-type material combine with holes of the p- type material along the junction between the two types of materials. This process results in the release of energy in the form of photons. These photons, in turn, stimulate other electrons and holes to combine, and a coherent beam is formed along the plane of the junction. The back surface of the diode is generally coated with a highly reflective metal and the front surface is polished to make it partially reflective.

Most semiconductor lasers are used to produce beams of infrared radiation. A major advantage of semiconductor lasers is that they can be made very small. Some types of semiconductor lasers can be made to flash on and off millions of times per second.

History

In the late 1950's, scientists began seeking ways to devise an optical maser--that is, a maser that would generate or amplify light (what is today called a laser). Preliminary studies were done by Charles H. Townes, inventor of the maser, with Arthur L. Schawlow, and by other scientists, including Gordon Gould, Nikolai G. Basov, and Aleksandr M. Prokhorov. The first successful laser was built in 1960 by Theodore H. Maiman of Hughes Research Laboratories. Maiman's laser contained a single large ruby crystal with two parallel surfaces silvered. The beam was emitted in a series of brief, intermittent pulses. Later in the same year, the first gas laser was operated at Bell Telephone Laboratories by Ali Javan and two

collaborators. This laser used a mixture of helium and neon and emitted a continuous beam.

Resistor

Resistor, a basic component of electric circuits. Resistors are used to produce heat, as in an electric toaster or furnace; to produce light, as in an incandescent light bulb; to provide an electrical bypass, as in the shunt in an ammeter; to regulate the electric power entering a device, as in a light dimmer or radio volume control; and to set voltages within an electric circuit.

The resistors used in heating and lighting applications are almost exclusively metallic. Such materials as platinum, tungsten, and Nichrome (an alloy of nickel, iron, and chromium) are commonly used. A wire-wound resistor consists of a coil of wire made of Nichrome or a similar material wound on a ceramic core and covered with a protective ceramic material.

There are several types of resistors used in electronic circuits. A carbon resistor is made of carbon mixed with a binding material such as clay and molded into a cylinder. A film-type resistor is made of a thin film of carbon, metal, or metal oxide on a ceramic base. Resistors in integrated circuits typically consist of a thin layer of semiconductor material or a thin metallic film.

Resistors may be either fixed or variable. Variable resistors having two terminals are called rheostats; those having three terminals are called potentiometers.

Semiconductor

Semiconductor, a material whose electrical conductivity is intermediate between that of a good conductor (such as copper) and that of an insulator (such as rubber). Most technologically important semiconductors are

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crystalline solids. They may be elements or compounds, either inorganic or organic Examples are the elements silicon, germanium, tellurium, and selenium; the inorganic compounds lead sulfide, cadmium sulfide, and indium antimonide, and the organic compounds anthracene, naphthacene, and phthalocyanine.

The semiconductor that is most important commercially is silicon, It is used in such solid-state devices as transistors and rectifiers. Miniature solid-state devices formed with silicon are essential components of integrated circuits (complex electronic circuits manufactured as a unit) that are used in a wide variety of electronic equipment. Silicon is also used in solar cells, which produce an electric current when exposed to light. Other important semiconductors are gallium arsenide, used in LED's (light-emitting diodes), and selenium sulfide and cadmium sulfide, which are used in photographic exposure meters.

Electrical Behavior

The usefulness of semiconductors lies chiefly in the nature of their electrical conductivity; to varying degrees, the conductivity increases with (1) an increase in temperature; (2) exposure to electromagnetic radiation; and

(3) the addition of small amounts of certain impurities called dopants. The sensitivity of semiconductors to temperature, radiation, and impurities is a consequence of the semiconductors' atomic structure.

In this discussion silicon and germanium, being typical semiconductors, are used as examples. Atoms of both silicon and germanium have four valence electrons; each atom therefore requires four additional electrons to complete its outermost electron shell, (An atom with a filled outer shell is particularly stable, and atoms tend to combine in such a way as to achieve complete outer shells.)

A model of a small portion of a silicon or germanium crystal is shown in the illustration. Each atom is at the center of a regular tetrahedron of four other atoms, which are called its nearest neighbors. This structure permits each atom to share one of its valence electrons with each of its nearest neighbors, and vice versa. Thus each atom has a complete outer shell of electrons, even though they are all shared, and each is bound to each of its nearest neighbors by a two-electron bond (the shared pair of electrons).

Sensitivity to Temperature

At all temperatures above absolute zero there is some heat energy, which causes the atoms of a silicon or germanium crystal to vibrate about their average positions and also causes some of the valence electrons (which are only weakly bound to the atoms) to escape. The higher the temperature, the more frequently such escapes will occur.

When an electron escapes, two charge carriers are formed—the electron itself (called a free electron) and a positively charged vacancy (called a hole) left in one of the two-electron bonds. Since the hole is positive, it can attract electrons (which are negatively charged) from a neighboring two-electron bond. If an electron in a neighboring two-electron bond has enough thermal energy, it can shift from its original bond to the hole. This action creates a new hole in the neighboring boring bond. This process can be repeated any number of times. Thus a hole can migrate through the crystal.

A hole and a free electron can recombine or unite whenever they meet to form a normal two-electron bond. In pure silicon or germanium this is the only mechanism by which a free electron or a hole can disappear.

The holes and the free electrons are the current carriers in silicon and germanium. Because the number of holes and free electrons increases with an increase in temperature, the electrical conductivity is greater at high temperatures than at low temperatures. At very low temperatures silicon and germanium behave like insulators, and at very high temperatures, like conductors.

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Sensitivity to Radiation

Electromagnetic radiation can also cause an electron to be ejected from a two-electron bond. If a photon of electromagnetic radiation has sufficient energy and passes in the vicinity of a two-electron bond in a crystal of silicon or germanium, it may be absorbed and an electron simultaneously ejected from the bond. The result is the creation of a free electron and a hole. Electromagnetic radiation thus increases the number of free electrons and holes (and therefore the electrical conductivity) in a crystal of silicon or germanium.

Sensitivity to Impurities

Two types of impurity atoms that have a profound effect upon the conductivity of silicon and germanium are those having either five valence electrons or three valence electrons.

Arsenic and phosphorus are examples of impurity atoms with five valence electrons. In a crystal of silicon containing a small amount of arsenic, the arsenic atoms randomly replace silicon atoms. Because the concentration of the arsenic is very small, the four nearest neighbors of any arsenic atom are silicon atoms. The arsenic atom forms four two-electron bonds with its four nearest neighbors exactly as a silicon atom would. Its fifth electron, however, cannot be part of a two-electron bond and the arsenic atom has a complete outer shell without it. Consequently this electron is much more easily removed from the atom than are any of the electrons in the two-electron bonds between arsenic and silicon atoms or between two silicon atoms. Thus, a silicon crystal containing arsenic contains many more free electrons at a given temperature than a crystal of pure silicon does.

Impurity atoms such as arsenic that provide free electrons in a semiconductor are called donor atoms. They cause the semiconductor to contain an excess of free electrons over holes. Such semiconductors are called n- type semiconductors. In such a semiconductor the greatest part of an electric current is carried by the free electrons, which in this case are called majority carriers. The holes are called minority carriers.

Boron and aluminum are examples of elements whose atoms have three valence electrons. When introduced as impurities they, too, greatly increase the electrical conductivity of silicon or germanium, but the mechanism of conduction is very different from that in the case just discussed. In a crystal of silicon containing a small amount of boron, the boron atoms substitute randomly for silicon atoms. Because a boron atom has only three valence electrons, it cannot form four two-electron bonds with its four nearest neighbors (which are silicon atoms because of the very low concentration of boron). Thus, one of the four bonds lacks a second electron. Although this structure is electrically neutral, there is a pronounced tendency to form four complete two-electron bonds. Consequently, the neutral boron atom tends to acquire an electron—either a free electron or an electron from a neighboring bond whose thermal energy is sufficient to allow it to jump. In either case the final result is a hole, which, as described before, is free to migrate through the crystal. Hence once again the presence of an impurity greatly increases the likelihood of the formation of charge carriers—in this case, predominantly holes.

Impurity atoms in semiconductors behave like boron in silicon are called acceptor atoms. They cause the semiconductor to contain an excess of holes over free electrons. Such semiconductors are called p-type semiconductors. In such semiconductors the greatest part of an electric current is carried by the positively charged holes. In this instance, the holes are called majority carriers and the free electrons, minority carriers.

If acceptor and donor impurity atoms are simultaneously present in a semiconductor such as silicon or germanium, then whichever one is present in the greatest concentration will determine whether the impure semiconductor is n-type or n-type semiconductor.

Thermoelectricity

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Thermoelectricity, electricity produced directly from heat. The production of electricity from heat is called the Seebeck effect, after the German physicist Thomas J. Seebeck, who discovered the phenomenon in the 1820's. Thermoelectricity arises in an electric circuit in which two dissimilar conductors or semiconductors are joined at their ends. When one of the junctions is at a different temperature than the other, a direct electric current will flow in the circuit. For a given thermoelectric circuit operating in a given temperature range, the magnitude of the current depends mainly on the temperature difference between the two junction—in general, the greater the temperature difference, the larger the current.

Thermoelectric circuits have been used in small thermoelectric generators to furnish power in remote areas and in space probes for radio transmitters and receivers and other devices that require relatively small amounts of electric power. The thermocouple, an important temperature-measuring device, also uses a thermoelectric circuit.

The Seebeck effect can be reversed—that is, when a direct current is sent through a circuit in which two dissimilar conductors or semiconductors are joined at their ends, heating will take place at one of the junctions and cooling at the other. This thermoelectric effect is called the Peltier effect, after the French physicist Jean C. A. Peltier, who discovered it in the 1830's. Small heaters and refrigerators whose operation is based on this effect have been developed.

Theory

An explanation of the Seebeck effect requires an understanding of the behavior of electrons inside a metal. Not all the electrons inside a metal are bound to specific atoms; some are free to move about. These free electrons behave like a gas. The density of the "free" electrons (the number per unit volume) differs from metal to metal. Consequently, when two different metals are placed in contact, their electron gases diffuse into one another. Because of the different densities of the electron gases and because electrons carry an electrical charge, the metals at the junction become oppositely charged. This difference in charge produces a potential difference across the junction. The extent of diffusion of the "electron gases" depends on the temperature. If the two junctions are at different temperatures, a potential difference will exist between the junctions and a current will flow.

Transformer

Transformer, a device that transfers electric energy from one circuit to another, usually with a change in voltage. Transformers work only with a varying electric current, such as alternating current (AC). Transformers are important in the distribution of electric power. They raise the voltage of the electricity generated at a power plant to the high levels needed to transmit the electricity efficiently. Other transformers reduce the voltage at the locations where the electricity is used. Many household devices contain transformers to raise or lower house-current voltage as needed. Television sets and stereo equipment, for example, require high voltages; doorbells and thermostats, low voltages.

How A Transformer Works

A simple transformer consists essentially of two coils of insulated wire. In most transformers, the wires are wound around an iron-containing structure called the core. One coil, called the primary, is connected to a source of alternating current that produces a constantly varying magnetic field around the coil. The varying magnetic field, in turn, produces an alternating current in the other coil. This coil, called the secondary, is connected to a separate electric circuit.

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The ratio of the number of turns in the primary coil to the number of turns in the secondary coil—the turns ratio—determines the ratio of the voltages in the two coils. For example, if there is one turn in the primary and ten turns in the secondary coil, the voltage in the secondary coil will be 10 times that in the primary. Such a transformer is called a step-up transformer. If there are ten turns in the primary coil and one turn in the secondary the voltage in the secondary will be one-tenth that in the primary. This kind of transformer is called a step-down transformer. The ratio of the electric current strength, or amperage, in the two coils is in inverse proportion to the ratio of the voltages; thus the electrical power (voltage multiplied by amperage) is the same in both coils.

The impedance (resistance to the flow of an alternating current) of the primary coil depends on the impedance of the secondary circuit and the turns ratio. With the proper turns ratio, the transformer can, in effect, match the impedances of the two circuits. Matched impedances are important in stereo systems and other electronic systems because they permit the maximum amount of electric power to be delivered from one component to another.

In an autotransformer, there is only one coil and both circuits are connected to it. They are connected at different points, so that one circuit contains a larger portion of the coil (that is, has more turns) than the other.

What are the different electrical insulators?

Even though electricity had been discovered long before, it was not until 1897 that scientists discovered electrons, upon which electricity is founded. Electrons are negatively charged particles that orbit the nuclei of atoms. In some materials, the electrons stick with their atoms rather than move around; these materials are called electrical insulators. The different electrical insulators include wood, plastic, glass, ceramic, cotton and air. The electrons in the atoms that make up these materials are tightly bound to their atoms.

In other materials, like most metals, the electrons (in this case called free electrons), do detach from their atoms and move around. These electrons are known as electrical conductors because they allow electricity to flow easily through these materials. In order to move, electricity needs to be transmitted by a conductor.

One thing electrical conductors are used for is the production of electricity by batteries, fuel cells and solar cells. Negatively charged electrons are drawn to anything that is positively charged, like the positive side of a battery. Just like water flowing downstream and turning a water wheel that happens to be in its way, electrons will flow through a battery and charge whatever load happens to be in their way (such as a light bulb).

How this works is as follows: The source of electricity (e.g. the battery) has a positive terminal and a negative one. When you connect a conductor (e.g. a copper wire) to the negative terminal, the electrons will flow through it. If you then attach a load (e.g. the light bulb) to the conductor, the electricity will power the load.

What is voltage?

Sometimes people get intimidated when thinking about electricity. They figure that they're not electricians and the whole field seems awfully complicated to them. Let's make it easier by picturing how water works. Imagine that you have a hose attached to a water faucet. You turn on the faucet and the high pressure at one end of the hose pushes out the water to the other end, where the pressure is lower. Now imagine voltage as being a measure of electrical pressure, with electric current traveling from one end of a wire to the other because there's more electric potential energy on one end than the other.

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Power surges, which increase the voltage in your electricity lines, can be caused by lightning striking nearby, increasing the electric pressure by millions of volts (even a surge protector won't be able to save your computer in this case); problems with the equipment of your utility company; faulty wiring in your house or apartment building; or when equipment that draws a lot of electricity, such as elevators, air conditioners or refrigerators, is suddenly turned on.

If the increase in electricity lasts for one or two nanoseconds (billionth of a second), it's known as a spike; but if it lasts three nanoseconds or more, it’s called a surge. Either one can put stress on your electronic components or can damage them severely, similar to how too much water pressure in a hose could cause the hose to burst. Although surge protectors do a fine job when dealing with fluctuations in voltage, you'll be better off unplugging your computer if a lightning storm is about to hit your neighborhood.

Arc, Electric

Arc, Electric, a sustained electric current passing from one solid electrical conductor, or electrode, to another through air or some other gas. The gas in the space between electrodes becomes an electrical conductor because the current ionizes it (gives the atoms of the gas an electric charge). An electric arc produces light and heat.

Furnaces that produce heat by means of an electric arc are used in foundries to melt iron. Another common use of the electric arc is in welding, where the heat of the arc fuses metals. Arc lamps using carbon electrodes create a bright, white light. They are used in some searchlights and were formerly widely used in the motion-picture industry for lighting scenes and in theater projectors.

An electric arc can be produced by several methods. In one common method, current is applied to two electrodes in contact with each other; when they are slowly pulled apart, an arc is formed.

Armature

Armature, the part of an electric generator or motor that contains the main current-carrying winding. The armature usually consists of a coil of copper wire wound around an iron or steel core. The coil and core are placed in a magnetic field produced by one or more permanent magnets or electromagnets. If the armature in a generator or motor is designed to rotate, it is called a rotor; if it is a stationary part, it is called a stator.

In a generator, either the armature or the magnet is rotated by an outside force (provided by a steam or water turbine or a gasoline or diesel engine) so that the armature coil cuts the lines of the magnetic field created by the magnet. This action produces an alternating current of electricity in the coil. This alternating current is transferred through slip rings (conducting metallic rings) connected to the ends of the coil to a set of brushes (stationary strips of metal) and conducted from there to the electric circuit where it is to be used. If direct current, instead of alternating current, is desired, a commutator (a ring divided into two insulated segments) is used instead of slip rings.

In an induction motor (the most widely used type of electric motor), an alternating electric current is supplied to the motor's electromagnets. The oscillating magnetic field produced by the magnets induces a current in the armature, causing it to rotate.

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Capacitance

Capacitance, the ability of an object or a system of objects to store an electric charge. All objects have this property to varying degrees. A familiar example of capacitance is the ability of a storm cloud to store electricity and then give it up in a bolt of lightning.

Quantitatively, the capacitance (C) of a body is defined as the ratio of the electric charge (Q) on the body to the voltage (V) of the body; that is, C = Q/V

The capacitance of a system depends upon the size, shape, and composition of the bodies in the system and their orientations with respect to one another. For example, a parallel plate capacitor—a system consisting of two identical flat, parallel metal plates separated by an insulator—has a capacitance given by the formula C = eA/d, where E is a constant that depends upon the insulator, A is the area of one of the plates, and d is the distance between the plates.

Capacitance is measured in farads (f). A parallel plate capacitor has a capacitance of 1 farad if a charge of 1 coulomb on each plate is needed to produce a voltage of 1 volt between the plates. The farad is too large a unit for practical purposes. Hence capacitance is commonly measured in millionths of a farad, or microfarads (mf).

Cathode Rays

Cathode Rays, in physics, a stream of electrons given off by the negative electrode, or cathode, of a vacuum tube. A cathode-ray tube (CRT) consists of an electron gun for emitting the electrons; deflecting plates for focusing the rays; and a screen, coated with a material such as zinc sulfide that will glow brightly when struck by the cathode rays. The picture tube in a television set is a form of cathode-ray tube. Other types are used in radar, in electronic measuring instruments, and as graphic-display terminals for computers and word-processing machines.

The rays leave the cathode at a very high speed, moving in a straight line across the tube. They can be bent from their straight path by a magnetic or electrostatic field.

Circuit Breaker

Circuit Breaker, an electric switch designed to break (open) an electric circuit automatically when the circuit is subjected to abnormal conditions. Once the problem in the electric circuit is corrected, a circuit breaker can simply be reset, unlike a fuse, which must be replaced.

A circuit breaker operates by separating two electrical contacts. In a typical home circuit breaker, the contacts are pulled apart by a spring-loaded lever when a catch restraining the lever is released. The catch can be released by either a thermal or an electromagnetic element. The thermal element consists of a bimetallic strip that bends as it is heated. The bending that results from overheating in the circuit, as when too much electricity is being drawn from it, is sufficient to release the catch. The electromagnetic element releases the catch by means of the magnetic force that arises when there is a surge of current through the element. Such a surge will occur with a short circuit.

Electric Meter

Electric Meter, or Watt-hour Meter, an instrument that measures the amount of electric energy used by a consumer. The meter is calibrated in kilowatt-hours. One kilowatt-hour is the amount of electric energy

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required to provide 1,000 watts of power for a period of one hour. (Ten 100-watt light bulbs left on for one hour consume one kilowatt-hour of electric energy.)

An electric power company uses electric meters to measure the amount of electricity consumed by each of its customers. The power company installs an electric meter near where its power lines enter a customer's building. It reads the meter periodically and charges the customer for the amount of electricity used.

The most common type of electric meter is essentially an electric induction motor that drives a series of geared wheels connected to indicators on the meter's face. This type of meter is designed for use with alternating current. It contains two electromagnets and a metal disk that is free to rotate between them. One electromagnet is powered directly by current from the incoming power lines; the other, by current drawn through the building's electrical circuits. The interaction of the magnetic fields produced by the coils causes the disk to rotate. Two permanent magnets near the disk's edge brake the disk in such a way that the speed of rotation is proportional to the amount of current drawn. As the disk rotates, it turns the series of geared wheels connected to the indicators on the meter's face.

Electronic watt-hour meters use solidstate circuits that produce electrical signals whose frequency or strength is proportional to the voltage and current being used. These signals are converted into energy measure ments recorded by mechanical or electronic indicators. Electronic watt-hour meters are generally more expensive than electromechanical models, but are more accurate. They can provide such features as the ability to record separately the energy consumed during different times of day and the ability to report meter readings by means of signals sent through the power lines to the power company.

Electric Shock

Electric Shock, a condition that occurs when there is a flow of electricity through that body. It is usually caused by contact with poorly insulated wires or ungrounded electrical equipment , by using electrical equipment while in contact with water, or by being struck by lightning. The severity and effects of electric shock depend mostly on the amount of current passing through the body and the duration of contact.

A slight, harmless shock produces only a jarring or startling sensation. Severe shocks produce muscle contractions, which lead to muscular spasms, paralysis, unconsciousness, or death. A fatal electric shock is called electrocution. Burns may occur where the current enters and leaves the body.

When an electric shock is caused by contact with electrical wires or equipment, the victim should be freed from the source of current immediately—either by shutting off the source (as by pulling a circuit breaker) or by separating the victim from the point of contact. Since the human body is a good conductor of electricity, the victim should not be touched with bare hands; dry, insulated gloves or a dry, nonconductive material (such as rubber or wood) should be used to push or pull the victim away from the source of current. In addition, the rescuer should stand on something dry and nonconductive.

A survivor of electric shock is often panicky with fear and is pale, trembling, and sweating. A doctor should be called immediately. First aid includes keeping the victim warm and in a horizontal position. If the victim stops breathing, mouth-to-mouth resuscitation should be given. Professional medical treatment may include treatment for burns and the administration of drugs and oxygen.

In most cases electric shock can be prevented by taking certain precautions, especially around the home. All appliances and switches should be in locations far from water and they should not be touched while standing in water or with wet hands. All electrical equipment should be permanently grounded. Frayed cords should be

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