stalled on the ceiling below. The effect of this short-circuit is shown in Fig. 7. Fig. 8 is from a photograph of an arc produced by a 250

volt, 10 ampere current between two wires which were touched together at a point where the insulation had been destroyed.

Constant-Potential System. A constant-potential system is one in which the voltage between the main supply wires is approximately the same at all points. Such a system is supplied by a constant

potential generator, which is of a character to furnish greater current as more devices, lamps, motors, etc., are put into use. Most in

candescent-lamp, motor- and street-railway circuits are of this type Fig. 9 shows such a 110-volt

and also many arc-lamp circuits.

Fig. 11. Diagram of a Three-Wire Circuit

A constant-current system is one in which the current is automatically maintained approximately the same regardless of the number and character of the power-consuming devices in use on the circuit. Devices, usually arc lamps, are connected in series and the generator is of a type which automatically increases the voltage or electric pressure as more lamps are turned on. Fig. 3 shows such a circuit and Fig. 10 is a picture of a generator used for series arc-lamp work.

Two-Wire System. A two-wire system is one having a single pair of distributing wires.

Three-Wire System. A three-wire system is a special form of a multiple system usually employing two generators. Fig. 11 shows such a system with two-wire branches to incandescent lamps.

Transformers. The chief advantage of alternating over direct current lies in the fact that electric power can by alternating

current be transmitted at a high voltage from the generating station to the point where the power is to be used, and there transformed with small loss to a voltage better suited to motors and lamps.

It is far more economical to transmit power at high voltages. It can be shown by electrical theory that by using twice as high

voltage, a wire only one-fourth as large is needed to transmit a given amount of power with the same percentage of loss on the transmission line. It should be remembered that if the voltage is doubled

the current need be only half as great in order to have the power delivered the same. But half as much current heats the wire which carries it only one-quarter as much and, since the heat dissipated on the wire is lost power, it is evident that economy requires that transmission of electric power be accomplished by high voltage and relatively small currents rather than the reverse. Thus it may be commercially possible to utilize a water power at a distance from a city to make high-voltage current which can economically be transmitted to the city over a line of rather small copper wires. It would never do, however, to carry this highvoltage current into buildings on account of the great fire risk involved, for the insulations in wiring cannot be made safe for high voltages. With direct current there is no very economical way to change a small current at high voltage into a larger current at a lower and safer voltage. With alternating current, however, this can be done very readily by means of a "transformer," a device which consists in its simplest form of two entirely separate coils of wire wound on the same iron core; in fact, a sort of double-coil magnet.

Fig. 12 shows two shapes which a transformer may take. In each diagram suppose an alternating current at 2,200 volts and 1

ampere is supplied to one coil of 100 turns. Then a current of 220 volts and 10 amperes will be induced in the other coil which has but 10 turns, that is, the voltage will be "stepped down" in the same ratio as the number of turns on the primary (power-supply) coil and secondary (power-using) coil, and if there were no power loss in the transformer itself, the current would be multiplied by the same ratio inverted. In the case illustrated there are one-tenth as many turns in the secondary coil as in the primary and, therefore, the secondary 2200X1 10

voltage will be

=220 volts, and the secondary current will be

approximately ten amperes for every ampere supplied to the primary.

If the power were supplied to the coil of the transformer having few turns, and used by the current induced in the coil having more turns, we should have a "step-up" in-. stead of "step-down" transformer.

It should be especially noted that the action of a transformer depends wholly on the principle that the rapid changes and reversals of current in one coil will induce currents in the other coil. Alternating currents are rapidly and systematically changing and reversing currents, and thus transformers

can be used to change the voltage and currents in a. c. circuits. Direct currents do not change in amount and direction and, therefore, transformers cannot be used at all on direct current.

Figs. 13 and 14 show the interior and exterior views of a small transformer of a type used for incandescent lighting. The case is designed to be filled with oil. Transformers are constructed of all sizes up to very large units, and are cooled either by air, air blast, oil, or by forced circulation of oil through the windings and core.

Alternating-current dynamos and transformers are constructed and connected in several ways which differ in regard to the number of partially independent circuits which they supply. An a. c. circuit in which there are but two wires from the generator to the trans

Fig. 15. Diagram of Single-Phase Light and Power Circuit

former or to lamps or motors is called a single-phase circuit. Fig. 15 shows the path of the current in a single-phase circuit which includes a generator G, switchboard with ammeter A and voltmeter V, transformers TT supplying power to lamps L and single-phase motor M. Single-phase circuits are used for lighting, both incandescent and arc, and for power.

The other most common type of a. c. circuit is called three-phase, and is also used both for light and power. The essential parts of such a system are shown in Fig. 16 where it will be noted that the dynamo has three slip rings, the circuit has three wires and the transformer has three coils in the primary T1, and thrce in the secondary T2. In a three-phase circuit each pair of wires carries what may be considered a separate alternating current which varies in each pair in regular cycles, so that the maximum current in each

1,

Fig. 16. Diagram of Three-Phase Alternating-Current Circuit

M

phase or pair of wires is reached at different instants of time. Such circuits are very widely used, especially for power, with what are known as three-phase induction motors.