Зображення сторінки

the circuit the same as before, 10 amperes. 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 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 tbe 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 necpssary to run two wires in all places, and from these wires leads are taken off and connected to the lamp.

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


Fia. 4.—incandescent Lamps in Devices connected in Multiple Circuit. 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 ampere, 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, c 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

[graphic][merged small]

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, one of very low resistance, connected directly between the two supply pipes, thus making a shunt, or by-passage, round the water-wheels. There would be nothing to limit the flow of water through this large pipe except the slight resistance to the passage of the water, and the ability of the pump to supply it. With an electric circuit on the "constant potential," or multiple system, there is an analogous effect when the wires are connected by a conductor of low resistance, or when they themselves come together. The dynamo is so constructed that it tends to keep the pressure constant. A tremendous current passes through the wires forming the short circuit, and if the dynamo is of sufficient size and there are no protective devices in circuit, the wires will heat until they actually melt.

It is commonly said that the electric current chooses the path of least resistance, but this needs a qualification to make it exact. With two paths more current will go through the path of low resistance than through the path of high resistance, but the total current will divide in exact inverse proportion. In Figure 5, a large pipe may be connected between A and B, and the flow of water in it will be proportional to the difference of pressure between A and B, and inversely proportional to the resistance of the connecting pipe. But the current through any other pipe between A and B, no matter how small, will also be proportional to the difference of pressure between its ends, and inversely proportional to its resistance, just as if the larger connecting pipe were not there. If the large pipe carries so much water that the pressure between A and B cannot be kept up by the pump, then of course the amount that will flow through any other pipe will be smaller than before the large pipe was connected. This, however, is because of the effect on the pressure and not because the water chooses the large pipe to the exclusion of the smaller.

The electrical multiple system may be elaborated by running from the mains, branches, and from the branches, taps, just as in a water or gas system branches or taps are taken from the main pipes. The dynamo is maintaining a pressure between the wires that lead from its terminals, and any branches that are taken from these wires are simply extensions of the mains, proportioned to the current they will have to carry. With a water or gas system, the free space outside, the air, may be looked upon as one of the pipes, and the devices used to keep up the pressure on the system are keeping up a pressure between the pipes and the free space. Electricity cannot flow in this space and it is thus necessary to provide the two wires.

« НазадПродовжити »