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zero. The hydraulic circuit used for illustration is not, of course, completely analogous to an electrical circuit, but it can be seen with this, too, that if a valve were closed just above the pump, the pressure might still be there, the steam might be in readiness pressing against the piston of the pump, but there would be no work done until the valve should be opened.

In the arc-light circuit, it is the number of lamps that determines the horse-power necessary to drive the dynamo. Each lamp is converting electrical energy into light and heat, which are given out to surrounding substances. The carbon rods are made to be a part of the circuit, and the mechanism of the lamp brings them together for the circuit to be formed through them. When the current is forced through the high resistance at the place of the contact, the carbon points become heated and at the same time the mechanism causes them to be slowly drawn apart. The current follows across the intervening space, because the heated air and the hot vapours from the carbons are conductors to some extent. The resistance for such a short distance is, however, so great, that an intense heat is produced and the carbon points and detached particles are raised to a high degree of incandescence. This incandescence represents the expenditure of a definite

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amount of energy; of a certain amount of coal burned under the boilers. If there is a current of 10 ampères through the lamp and a difference of pressure of 45 volts between the point where the current enters the lamp and where it leaves it, then 450 watts, or about 6 horse-power, is a measure of the rate at which work is being done at the lamp.

The series system of lighting offers many advantages when arc lamps are used. With these, a high potential circuit is not seriously objectionable, and when the pressure at the dynamo may be 2500 or 3000 volts, a comparatively small wire may be used to transmit the energy required for the 50 or 60 lamps in a circuit. For example, 5 miles of No. 6 copper wire has a resistance of about 101 ohms. If this wire is carrying an arc-circuit current of 10 ampères, there is required 105 volts (E=CR=10×101=105) pressure to force the current through the resistance of the wire alone. This means that the rate of expenditure of energy, or rate of work, in the wire is 1050 watts (watts = EC=105 × 10=1050) or about watts 1050 746 746

1.4 horse-power (horse-power =

=

1.4).

This, however, would be only 4.2% of the total energy of the circuit if the pressure were 2500 volts at the dynamo. But suppose the total pressure at the dynamo to be 105 volts and the current forced through

the circuit the same as before, 10 ampères. The loss in wasteful heating of the wire would be exactly the same, for it depends only upon the resistance and the current forced through through this resistance. The total power of the circuit would be now, however, only 1050 watts (105 x 10 = 1050) so that the loss in the wire is 100%. The whole of the power would be spent simply in forcing the current through the resistance of the wire. There would be nothing at all left for useful work. If any lamps were put in circuit they could not have the full current. There would be an added resistance, and the current would immediately be less by an amount that would compensate for the work added.

In street-lighting, where the lamps are so widely distributed, there is another advantage in the series system. Since the lamps are connected in circuit one. after the other, only one wire need be run to the lamp, and another away from it, to the next. Where the multiple system is used, it is necessary run two wires in all places, and from these wires leads are taken off and connected to the lamp.

to

The series system is also satisfactory for incandescent lights in the streets, but it is unsuited for incandescent lamps for interior lighting. With lamps in all sorts of places as they are in a building, the voltage, or pressure, must be kept low, and each

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lamp must be independent of the others. If there are many lamps in series the pressure that is necessary is difficult to make safe; and the necessity, too, in the series system, of providing equivalent resistances or shunting" devices for the lamps turned off, makes an awkward complication of apparatus. Incandescent lamps, and in general all devices except arc lamps, are consequently run on the multiple system, and even

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FIG. 4. Incandescent Lamps in

Multiple Circuit.

arc lamps for interior lighting are in many places being thus connected.

Devices connected in multiple are indepen

dent of one another, each receiving at its terminals the full pressure of the system, which always remains the same whether few or many of the lamps or other devices are being operated. It is here the current that changes, each lamp adding its one-half ampère, it may be, to the total load of the dynamo. The electrical circuit operated in this way is shown in Figure 4.

The hydraulic analogue may be represented as in Figure 5. Here, the pump keeps up between the two pipes, A and B, a constant pressure, and between the pipes are connected the water-wheels, c, d,

and e.

It will be seen that the wheels are quite independent of one another. e may be shut off by turning a stop-cock in the small pipe leading to it, without interfering at all with d and e. If the pressure between A and B is kept constant, the only effect caused by turning off one of the wheels is to diminish by one-third, the amount of water delivered by the pump.

The power expended in the whole circuit is here, as in the series system, the pressure multiplied by

B

FIG. 5. Water-wheels in Multiple.

the current. In the series system the current remained always the same, while the pressure varied according to the number of devices operated, but here there is always the comparatively low pressure required by one device, and the current from the pump or dynamo varies according to the number of devices operated.

The effect of a "short circuit" on an electric system may be appreciated by imagining a large pipe,

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