512 MAGNETISM AND ELECTRICITY used. Thus the transformer has permitted the commercial use of alternating current. A transformer which increases the voltage is called a step-up transformer. Transformers in which the number of turns in the primary is greater than that in the secondary decrease the voltage and are called stepdoivn transformers. Pig. 247. Distribution transformers. (Courtesy of the Allis- Chalmers Manufacturing Company.) In large transformers, the insulated coils are usually surrounded by oil to prevent short circuits and to keep the transformers cool. (The oil is cooled by radiators.) The coils in smaller transformers are frequently packed in pitch or tar for the same reason. In distributing current through a city, it has been found most economical and safe to transmit the power at 2000 to 4000 volts. Substations are generally located outside the city to transform higher voltages to the above voltages. Transformers are also located at points where this current is distributed to business houses or groups of dwellings to transform the voltage down to 220 or 110 volts. High voltage is dangerous, but the ordinary household current of 110 volts is not so hazardous, except when unusually good contacts are made, such as when one stands on a wet floor or in a bathtub and touches a poorly insulated electric appliance. Small transformers used for doorbells or toy railroads step down the voltage again to 6 or 8 volts. Transformers which have just one turn in the secondary, using a wire of large diameter, produce a current of high amperage but low voltage. Such a current is used in electric welding because it enables sufficient heat to be obtained at the point of contact of two metals which form part of the circuit to weld them together; the heat is the product of the square of the current times the resistance. proportional to A Choke Coil Acts by Self-inductance. A choke coil, such as is used in radios or fluorescent light controls, consists of a coil of wire through which the current flows. The current sets up magnetic lines of force that cut the turns of wire in the coil and thus induce a voltage within the circuit that opposes the original volt-
ELECTROMAGNETIC INDUCTION 513 age. This induced voltage is said to be self-induced. The induced electromotive force thus acts to choke, i.e., prevent the current from rising rapidly to its final value, and it also prevents the current from stopping immediately when the electromotive force is removed from the circuit. The Telephone Is an Application of Electromagnetic Induction. The telephone was invented by Alexander Graham Bell (1847-1922) in 1876. Bell's first telephone was a crude device, in which the receiver and transmitter were essentially the same. The diaphragm was made of goldbeater's skin, an animal membrane which was attached to a permanent bar magnet by a kind of lever. This magnet was surrounded Fig. 248. The simplest telephone circuit. (Courtesy of the American Telephone and Telegraph Company.) by a coil of wire, in which a current was induced when the magnet was moved by the vibrations of the diaphragm. In a later telephone, in which the transmitter and receiver were the same, a horseshoe magnet in the transmitter had two coils wound on its ends, from which wires were connected to similar coils in the receiver. A battery maintained the flow of current through the coils. Sounds caused the iron diaphragm in front of the magnet in the transmitter to vibrate. This caused a fluctuation in the current flowing through the coils, which was transferred to the diaphragm at the receiver end. Since 1877 there have been more than ninety types of transmitters and more than sixty types of receivers designed and used. In 1877 Thomas Edison patented the carbon transmitter, which was so arranged that vibrations in the diaphragm produced fluctuations in the current. Behind the mouthpiece of the transmitter is a metal box containing two insulated disks between which are packed fine carbon granules. The current passes through these granules and varies with the changes in pressure on the carbon resulting from motion of the diaphragm. Thus the current passing through the wire is not induced as in the earlier type of transmitter but is modulated in intensity.