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discarded and unused sockets covered or sealed. Before making any electrical repairs fuses should be removed or circuit breakers turned to the "off" position. Bare wires which could possibly be carrying current should never be touched.

Electric Switch

Electric Switch, a device for completing and breaking an electric current, or for changing the path of a current. Electric switches are among the most common types of control devices and are in wide use wherever electricity is available. There are two basic types of switches, electromechanical and electronic. Only electromechanical switches are discussed here; electronic switches are described in Electronics, section “Some Basic Electronic Devices and Circuits,” subtitle Logic Circuits.

In its simplest form, a switch consists of two contacts, one fixed and one movable. When the contacts are brought together, the switch and the circuit are closed and current flows through the circuit. Operating the switch to disconnect the contacts opens the circuit and stops the flow of current. This type of switch, known as a single-pole switch, is commonly used in the home for turning lights on and off.

Switches are available in many types and sizes for a vast number of uses. Household wall switches include the familiar snap-action type, which contains a spring to give positive opening and closing action, and the mercury switch, in which a drop of mercury in a sealed glass tube carries current from one contact to the other when the switch is closed. The mercury switch is also used in thermostats.

Three-way switches are of two different types. In one, current is directed to one or both filaments of a doublefilament (threeway) light bulb. The other type is used in pairs to control a single light from two locations, such as the top and bottom of a staircase.

In one type of photoelectric switch, light striking a photoelectric cell generates a current that causes an electromagnet to hold the switch open. When the light fails or is interrupted, the switch closes. Such switches are used in some types of automatic door openers and for automatic control of outdoor lighting.

Membrane, or touch, switches are commonly used in electronic calculators and other low-voltage devices containing microelectronic chips. The switching circuit is printed in two parts; each part is on a plastic film and the two films are separated from each other by a sheet of insulating material. The circuit is closed by lightly pressing one of the plastic films (or a key located directly above the film) so that the two parts make contact through an opening in the insulating sheet.

Electrode

Electrode, a conductor through which an electric current enters or leaves a solution or other medium in an electrical device such as a battery, electrolyt ic cell, or electron tube. In some devices electrodes are also called poles or plates.

The electrodes of a battery are separated by a solution containing ions (electrically charged atoms or groups of atoms). One of the electrodes—the negative electrode—undergoes a chemical reaction that gives it excess electrons. The other electrode—the positive electrode—undergoes a chemical reaction that removes electrons. When the two electrodes are connected by an external electrical circuit, the excess electrons flow from the negative to the positive electrode.

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Electrolytic cells and electron tubes are connected to an external source of electrical power, such as a battery or dynamo. The power source moves electrons into one of the electrodes (making it the negative electrode), withdraws electrons from the other (making it the positive electrode), and causes a current to flow through the medium between them.

The electrode through which negative charge enters an electrical device is called the cathode; the electrode through which negative charge leaves is called the anode. The negative electrode of a battery is thus the anode, whereas the negative electrode of an electrolytic cell is the cathode.

Electroscope

Electroscope, an instrument used for detecting electric charges or for measuring small electric voltages or currents. It is used in the laboratory—for experiments and demonstrations—and in industry, where it is connected to meters and other sensing devices. A simple electroscope consists of a glass jar in which two strips of gold leaf are suspended from a metal rod that conducts electricity. The rod, which enters the jar through a stopper made of a material that does not conduct electricity, has a metal knob on the end outside the jar.

The strips of gold leaf hang straight down when they are not charged. When a charged body is brought near the metal knob, both strips acquire a like charge (that is, they both become negative or both become positive). As a result, they repel each other and spread apart to form an inverted V. The electroscope is then charged. If an oppositely charged body is brought close to the knob, the charge on the strips is neutralized, and they again hang straight down. The electroscope is discharged.

By using a specially calibrated microscope to observe the movement of the strips, a scientist can measure the voltage of the charge in microvolts (millionths of a volt). When the electrical capacities of both the electroscope and the body producing the charge are known, electric currents moving through ionized air can be measured Even when the capacities are not known, these currents can be detected. Therefore, the electroscope is used for detecting X rays, cosmic rays, and radiation from radioactive material. These rays ionize the air and pass through it as a kind of electric current. The current either charges or discharges an electroscope.

Fuel Cell

Fuel Cell, a device that converts the chemical energy of a fuel directly into electrical energy. In contrast to a steam-powered generator system, the fuel cell does not first convert chemical energy into mechanical energy. Fuel cells, like electric cells (batteries), have no moving parts. Unlike electric cells, fuel cells use outside materials in producing an electric current. Fuel cells are very efficient in converting the chemical energy of a fuel into electrical energy. However, for most applications they are too expensive to compete with conventional methods of producing electricity. Their main use is on spacecraft and to provide power at remote locations on earth.

A fuel cell uses a fuel, usually hydrogen, and an oxidizer, usually oxygen (or air), to produce direct-current power. A typical fuel cell contains two electrodes, in the form of metallic screens, separated by a material saturated with an electrolyte, such as potassium hydroxide. Hydrogen is supplied to one screen, oxygen to the other. Chemical reactions between the electrolyte and each of the gases creates a voltage between the electrodes. Water is formed as a by-product of the reactions. The fuel cells used on American space shuttles weigh about 200 pounds (90 kg) and can produce up to 12,000 watts of power at 27.5 volts. Each space shuttle carries three cells.

The first research on fuel cells began in the mid-1800's, but promising models did not appear until the early 1960's. The first practical application of fuel cells was as the main power supply in Gemini spacecraft.

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Fuse, Electric

Fuse, Electric, a protective device for breaking an electric circuit. A fuse consists essentially of a metal strip or wire that melts at a lower temperature than the wire and other components in the rest of the circuit. The fuse is connected in series with the source of electric power. When the current flowing in the circuit becomes too strong, it heats the fuse so that it melts, breaking the circuit.

Household fuses are rated according to the amount of current they are designed to carry before blowing (melting). The most common causes for blown fuses in the home are (1) short circuits and (2) the operation of too many appliances at one time on the same circuit. Fuses have been largely replaced in many applications by circuit breakers.

Galvanometer

Galvanometer, an instrument used to indicate the presence, direction, or strength of a small electric current. The typical galvanometer is a sensitive laboratory instrument used mainly to detect and compare currents.

The galvanometer makes use of the fact that an electric current flowing through a wire sets up a magnetic field around the wire. In the galvanometer, the wire is wound into a coil. When current flows through the coil, one end of the coil becomes a north magnetic pole, the other a south magnetic pole. When a permanent magnet is placed near the coil, the two fields—the one from the coil and the one from the magnet—interact. The like poles will repulse each other and the unlike poles will attract. The amount of attraction and repulsion increases as the strength of the current increases.

In the moving-magnet galvanometer, the permanent magnet is a needle (much like a compass needle) mounted on a pivot and surrounded by the coil. In the moving-coil galvanometer—the most common type—the coil is mounted on pivots or suspended by thin metal strips. The coil lies between the poles of a permanent magnet in such a way that it rotates when current flows through it. The direction of the rotation depends on the direction of the current through the coil, and the amount of rotation depends on the strength of the current. A galvanometer is often used to indicate when the current in a circuit has been reduced to zero, as in the operation of the Wheatstone bridge, a device for measuring electrical resistances precisely.

A moving-coil mechanism similar to that used in a galvanometer is used in some ammeters. Like the galvanometer, these instruments measure the strength of a current but they can handle a stronger current; unlike the galvanometer, they cannot indicate the current's direction. A moving-coil mechanism is also used in some voltmeters (which measure the voltage in a circuit) and ohmmeters (which measure the resistance in a circuit). In some instruments, a selector switch connects the moving-coil mechanism to different internal circuits so that a single mechanism can be used in making all three types of measurements.

The principle upon which the operation of the galvanometer is based was discovered in 1820 by Hans Christian Oersted when he observed that a magnetic needle could be deflected by an electric current. The first galvanometer was made by Johann Schweigger in 1820. In 1882, Jacques Arsene D'Arsonval introduced the moving-coil galvanometer. Edward Weston made important improvements to the device a few years later.

Grounding, Electrical

Grounding, Electrical, the connecting of electrical equipment and wiring systems to the earth by a wire or other conductor The primary purpose of grounding is to reduce the risk of serious electric shock from current leaking into uninsulated metal parts of an appliance, power tool, or other electrical device. In a properly grounded

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system, such leaking current (called fault current) is carried away harmlessly. Grounding is also used in manufacturing industries to prevent accumulation of hazardous static electrical charges.

Although most electrical systems have fuses or circuit breakers for protection against a major fault current or short circuit, the human body may be fatally shocked by a current of less than one amperewell below the point at which a fuse or breaker will operate. Grounding helps prevent such a hazard from occurring. In some cases, however, as when a person handles an electrical device while standing on a wet surface, there is a risk of fatal shock from a leaking current even from a properly grounded electrical circuit. For protection against this danger, a safety device called a ground-fault interrupter can be installed in the circuit. This device, so sensitive that it can detect leakages as small as 5 milliamperes, immediately disconnects the circuit when a leakage occurs.

In most homes, the wiring system is permanently grounded to a metal pipe extending into the house from an underground water-supply system or to a metal rod driven into the ground. A copper conductor connects the pipe or rod to a set of terminals for ground connections in the home's electrical service panel. In wiring systems that use electrical cable sheathed in metal, the metal sheathing usually serves as the ground conductor between wall outlets and the service panel. In wiring systems that use plastic-sheathed cable, an extra wire is used for grounding.

An appliance designed to be grounded is equipped with a three-wire cord and a plug having three prongs that is intended to be plugged into a matching outlet. The third wire and prong provide the ground link between the metal frame of the appliance and the ground of the wiring system. Toasters and other appliances having exposed heating coils should not be grounded, because grounding actually increases the risk of electric shock. Double-insulated power tools and small appliances have specially insulated housings that eliminate the need for grounding. These are designed so that no exposed part of the device will be electrically live even if the internal insulation fails.

Protection against lightning by lightning rods and other means is a form of grounding.

Induction Coil

Induction Coil, a device for converting low-voltage direct current (DC) into high-voltage alternating current (AC). The coils are used chiefly in the electrical systems of automobiles and to operate X-ray tubes.

A typical induction coil has a core of soft iron, a primary coil, and a secondary coil. The primary coil consists of a few turns of fairly heavy wire around the core; the secondary consists of many turns of fine wire around the primary. The primary coil forms part of a circuit called the primary circuit that includes a direct current source and a circuit breaker, or interrupter.

When the primary circuit is closed, direct current flows through the primary coil, producing a magnetic field. As the magnetic field builds up, it induces an electric current in the secondary coil. At the same time, the iron core becomes magnetized. The magnetized core draws the interrupter away from a metal contact, breaking the primary circuit. The direct current in the primary coil ceases and the coil's magnetic field collapses, again inducing an electric current in the secondary coil, only in the opposite direction. Simultaneously, the core loses its magnetism and releases the interrupter, which is pulled back against the contact by a spring. The cycle continues to repeat rapidly, supplying an alternating current at the terminals of the secondary coil. The voltage in the secondary coil is higher than in the primary coil because of the greater number of turns in the secondary coil.

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A capacitor, or condenser, is often used with an induction coil. The capacitor prevents sparking between the interrupter and contact by briefly storing the electric charge that would otherwise jump the gap between them.

Leyden Jar

Leyden Jar, an early and simple form of capacitor (electric condenser), a device for storing electricity. It was invented about 1745 by E. G. von Kleist of Prussia and, independently, by Pieter van Musschenbroek of the University of Leiden in Holland. The first detailed experiments with it were done by Musschenbroek. The jar is used today only for physics laboratory demonstrations. It consists of a glass container covered with tinfoil inside and out for about one-half of its height. A brass rod passes through a cork or wood stopper and is connected to the inner tinfoil by a chain.

The jar is charged by connecting a brass ball at the top of the rod to a body with an electric charge. At the same time the outer tinfoil is grounded (connected to the ground), usually by being held in the hand. The inner tinfoil receives an electric charge which induces an opposite charge on the outer tinfoil. The outer charge permits a greater charge on the inner tinfoil, which then induces a stronger charge on the outer tinfoil. Thus the charges can be built up until the jar reaches the limit of its capacity.

The glass keeps the charges apart. But when a conductor is run from the brass rod to the outer tinfoil, the jar will discharge with a spark, producing an oscillating current.

Ohm's Law

Ohm's Law, a law of physics. It states that in an electrical conductor the ratio of potential difference (voltage) to current is constant. For example, if the terminals of an electric battery are connected to an electric lamp and the voltage output of the battery is then decreased by 20 per cent, the amount of current flowing through the lamp will also be reduced by 20 per cent.

Ohm's Law was derived experimentally by the German physicist Georg Simon Ohm in 1826. It is expressed by the following equation:

V=I X R

In this equation V represents the potential difference between one end of the conductor and the other (that is, the voltage applied to the conductor); I is the current flowing through the conductor; and R is called the resistance of the conductor. If V is given in volts and I is given in amperes, R will be in ohms.

The law offers a simple method of calculating the voltage, current, or resistance in a conductor when two of these three quantities are known. For example, if the direct-current voltage applied to an electric light bulb is 120 volts and the filament in the bulb has a resistance of 240 ohms, the current flowing through the filament isI = V/R = 120 volts/240 ohms = 0.5 ampere.

Ohm's Law is valid for metallic conductors (for example, copper and tungsten) in direct-current circuits as long as the current is relatively low. High currents will heat a metallic conductor and cause its resistance to change, so that the ratio of voltage to current in the conductor will also change. Ohm's Law holds for a complete directcurrent circuit as well as for any part of the circuit, provided that l is the total current flowing between the points across which the voltage, V, is measured.

In alternating-current circuits, Ohm's Law seldom is valid because the current flow is affected not only by resistance but also by factors known as inductance and capacitance.

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Oscilloscope

Oscilloscope, an instrument that produces a visual image of an electrical signal. The image is produced on a luminous screen similar to the picture screen in a television receiver. The screen forms one end of a cathode-ray tube. By observing the visual image of an electrical signal, scientists or technicians can determine at a glance such things as the strength of the signal, the wavelength (or frequency) of the signal, whether any undesired phenomena are affecting the signal, and whether the source producing the signal is functioning properly.

The electrical signals displayed on the oscilloscope are not necessarily of electrical origin. A large number of physical phenomena—such as light, radio waves, and sounds—can be converted to electrical signals suitable for display on an oscilloscope. Sounds, for example, can be converted into electrical signals by a microphone.

If desired, the images displayed on the oscilloscope screen can be photographed. An oscilloscope equipped with a camera is sometimes called a cathode-ray oscillograph.

The German physicist Karl Ferdinand Braun invented the oscilloscope in 1897.

Piezoelectricity

Piezoelectricity, electricity generated by certain crystals and ceramics when they are deformed by mechanical pressure. The effect is reversible, that is, an electric current can produce deformation in these crystals and ceramics. Crystals showing the piezoelectric effect include quartz and Rochelle salt. The most commonly used piezoelectric ceramic is barium titanate.

The piezoelectric effect is particularly useful in converting mechanical strain into electrical impulses and converting electrical impulses into mechanical strain. Piezoelectric crystals and ceramics are used in phonograph cartridges, microphones, electronic oscillators, and measuring instruments.

Potentiometer

Potentiometer, an electrical device that measures potential difference between two points in a circuit by comparison with a standard battery of known potential difference. The name is also applied to a variable resistor that provides varying amounts of potential difference.

Resistance, Electrical

Resistance, Electrical, the opposition that a material offers to the passage of an electric current. Resistance converts part of the electrical energy of the current into heat. The property of resistance has important applications in electrical and electronic devices.

Resistance is measured in units called ohms. A conductor has a resistance of one ohm when a voltage of one volt across it causes a current of one ampere. Stated as a formula, the relation between resistance, voltage, and current is

R=E/I

where R is the resistance in ohms, E the voltage in volts, and I the current in amperes.

In general, the resistance of a metallic conductor at a given temperature is constant. Thus the current through the conductor changes in proportion to the voltage. This relation is called Ohm's Law. The resistance of diodes,

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transistors, and other semiconductor devices usually depends in part on the voltage or current applied to them. Some substances lose all electrical resistance when they are cooled to very low temperatures.

Rheostat

Rheostat, a device used to regulate an electric current by increasing or decreasing the resistance of the circuit. Some common uses of the rheostat are to dim lights, to control the speed of an electric motor, and to control the volume of a radio. The accompanying diagram shows a simple rheostat. Current flows into the resistance coil and from the resistance coil through the slider. When the control knob is turned, the slider moves along the coil; the amount of resistance is thus changed by varying the length of the current's path through the resistance coil.

Static Electricity

Static Electricity, a motionless electrical charge, as distinguished from current electricity. The theoretical aspects of static electricity are discussed in the article Electricity, subtitles The Nature of Electricity: Static and Current Electricity; and How Electricity Is Produced: Static Electricity. This article is concerned with some of its occurrences, hazards, and uses.

When the relative humidity is low, as in a region with a dry climate or in a heated building during winter, static electricity is sometimes encountered. Under such dry conditions, a person walking across a carpeted floor can build up a static charge because of the friction of his shoes on the carpet. When the person touches an object that is uncharged or that has an opposite charge, the static charge is released quickly. Although annoying, such static shocks are usually harmless. Another common, but dangerous, example of the discharge of static electricity is lightning. Lightning occurs when a storm cloud builds up very large static charges.

In manufacturing industries, static electricity can be both a serious nuisance and a major safety hazard. In a paper mill, for example, static electricity may cause sheets of paper to cling together, creating delays and extra expense for the manufacturer. Large static charges may develop on drive belts between motors and machinery; if not diverted through proper grounding, the built-up static charges may discharge suddenly and cause damage to equipment. In areas containing explosive materials, great care must be taken to prevent static discharges, since a spark could set off a violent blast.

Under the proper conditions, static electricity can be very useful. Electrostatic precipitators are used to trap dust and other air pollutants in some factories and home heating and air conditioning systems. A widely used business machine, the electrostatic copier, provides quick and accurate copies of documents and drawings. Very large static charges, for use in nuclear research and in special X-ray work, can be produced by special machines such as the Van de Graaff generator.

Superconductivity

Superconductivity, the property that certain materials have of losing all resistance to an electric current. Such a material loses its electrical resistance when cooled below a temperature called the material's critical (or transition) temperature. Many pure metals and alloys are superconducting, but only at temperatures near absolute zero (0 K, or -273.15° C. *-459.67° F.+). Some niobium alloys have critical temperatures near 20 K (-253°

C.). Several synthetic copper-oxide materials have higher critical temperatures; one such material, containing thallium, has a critical temperature near 125 K (-158° C.).

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The most important use of superconducting materials, or superconductors, is for making powerful electromagnets. A common practical application of such magnets is in magnetic resonance imaging (MRI) devices, used for medical diagnosis. The operation of various types of research equipment also depends on superconducting magnets. The Tevatron, a powerful particle accelerator at the Fermi National Accelerator Laboratory in Illinois, uses hundreds of such magnets for its operation. Superconducting magnets are usually made of niobium alloys that can carry a very strong electric current without losing their superconductivity. Although the electromagnets must be cooled with liquid helium, requiring complex cooling apparatus, they consume much less electrical power than comparable conventional electromagnets.

Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes while he was studying the conductivity of mercury cooled to very low temperatures. In 1957, John Bardeen, Leon N. Cooper, and John R. Schrieffer developed a theory that successfully explains superconductivity in terms of an interaction between electrons that prevents them from scattering when they flow through a material at or below its critical temperature.

In 1986, J. Georg Bednorz and K. Alex Mller discovered superconductivity in a compound of lanthanum, barium, copper, and oxygen at a temperature near 35 K (-238° C.). This discovery, for which Bednorz and Mller were awarded the 1987 Nobel Prize in physics, sparked a flurry of research focused on copper-oxide materials, and, within little more than a year, scientists found similar materials having a critical temperature as high as 95 K (-

178° C.).

These new materials showed great potential for a variety of uses, but were difficult to produce commercially. In the 1990's, researchers discovered ways to craft relatively high-temperature superconductors into useful magnetic components for research and for medical diagnostics.

Thermocouple

Thermocouple, an electrical device that responds to a difference in temperature by producing an electric current. Thermocouples are used as measuring instruments and as control devices. Thermocouples are simple and rugged, can be used over a wide range of temperatures (from -200° C. to 1,600° C.), and permit great precision. For these reasons thermocouples are used to measure temperatures inside furnaces and jet engines and in laboratory experiments. In some gas appliances, thermocouples are used as safety switches to control the gas supply.

The thermocouple is based on the Seebeck effect, named after its discoverer, Thomas J. Seebeck. In its simplest form, the thermocouple consists of two wires of dissimilar metals or alloys joined at their ends, with a potentiometer (or a voltmeter) connected in one side of the circuit. The diagram shows such a thermocouple using wires of iron and constantan (an alloy of 60 per cent copper and 40 per cent nickel).

The measuring junction is placed in the environment whose temperature is to be measured. For precision work, the reference junction is kept at a fixed, known temperature (for example, by being placed in an ice bath). If great precision is not required, the reference junction may be left at room temperature, which is known only approximately. The potential difference between the two junctions, as shown by the potentiometer or voltmeter, is used to find the temperature of the heated junction, usually from a table in a handbook.

When used as a control device in a gas appliance, the thermocouple is mounted so that its measuring junction is heated by a pilot light. The electric current generated is sent through an electromagnet. As long as the current flows, the electromagnet holds open a valve that allows gas to reach the appliance. If the pilot light goes out, the measuring junction cools off, no current flows, and the electromagnet releases the valve, shutting off the gas.

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A number of thermocouples connected in series make up a thermopile. Thermopiles are more sensitive than individual thermocouples (some thermopiles can measure temperature differences as small as a millionth of a degree). Thermopiles are used for measuring the temperature of radiation from stars, for detecting the amount of heat produced by living tissues, and in other situations where highly sensitive temperature-measuring devices are required.

Transducer

Transducer, in electronics, a device that converts electric energy into some other form of energy, or vice versa. Transducers are used in sound systems, in video equipment and in many measuring devices. A transducer used in a measuring device produces an electric current proportional to the strength or intensity of the physical quantity—such as heat, light, or mechanical stress—to which the transducer is subjected. Piezoelectric crystals (crystals that produce an electric current when deformed) and photoelectric cells are transducers; so are loudspeakers, which produce sound from electric energy.

Transistor

Transistor, a solid-state device used in electronic circuits primarily as an amplifier or switch. Almost all modern electronic equipment makes use of transistors. In miniaturized form, they are essential components of computers. Transistors are made of semiconducting materials, such as silicon or germanium. They are produced in a variety of sizes and shapes. A conventional transistor is enclosed in a protective casing and has three electrical leads. Transistors that are components of an integrated circuit are formed on the surface of a single chip of semiconducting material and can be microscopic in size.

The transistor was invented by William Shockley, John Bardeen, and Walter H. Brattain of the Bell Telephone Laboratories in 1948.

Van de Graaff Generator

Van de Graaff Generator, an electric induction machine used in nuclear research and cancer therapy. The Van de Graaff generator is a particle accelerator, a device for increasing the velocity, and therefore the energy, of atoms and subatomic particles. It consists of a hollow metal sphere mounted on a hollow column that insulates the sphere from the ground.

A moving cloth belt inside the column carries electrons from a battery up to the sphere, where they are deposited. As the electrons accumulate, the sphere obtains an increasingly higher electric charge. Finally, the difference in electric potential between the sphere and the ground is so great that the charge rushes down through the column back to the ground. The electrons have a high velocity, and can be channeled into other apparatus to bombard various materials to release other subatomic particles or to produce X rays.

The generator was invented in 1928 by Robert Jemison Van de Graaff of the Massachusetts Institute of Technology.

Wheatstone Bridge

Wheatstone Bridge, a device for measuring electrical resistance. One form of Wheatstone bridge is shown in the illustration. When the bridge is connected in an electrical circuit, part of the current flows to the object whose resistance is unknown (a light bulb in the illustration), and part flows to the resistor of known resistance. If more

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current flows through one side of the circuit than the other, the galvanometer shows the difference. The sliding contact is then moved along the wire until current flows equally along both sides of the bridge and the galvanometer shows zero.

When the bridge is thus balanced, the unknown resistance is calculated by a formula. The formula is: X = RD'/D (X is the unknown resistance. R is the known resistance. D is the distance from the key to the right end of the meter stick. D' is the distance from the key to the left end.)

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