Part 11

The wiring for incandescent lamps is carried out in an entirely different way, which you can see by comparing Fig. 25 A with Fig. 25 which shows the wiring for arc-lamps.

This is called connecting in "multiple arc."

You will notice that the two wires running out from the dynamo (which are called the main wires) do not form one continuous loop as in the arc-light system, but that a smaller wire is attached to one of the main wires and then connected with the screw-ring in the lamp-socket; then another wire is connected with the button in the socket and afterward to the other main wire. Every lamp forms an independent path through which the current can travel back to the dynamo.

Now, if we turn one of these incandescent lamps out, we simply shut off one of these paths and the electricity travels through the other lamps, and, if we wish, we can turn out all the lamps but one and there will still be a way for the electricity to go back to the dynamo.

In the arc-lamps we must have a very high number of volts pressure, because the electricity has only one path, and it all has to pass through the first and other lamps till it comes to the last one. In the incandescent light the electricity has as many paths as there are lamps, so we only need to keep one certain pressure in volts in the main wires all the time. This pressure is even all the way through the main wires, and, therefore, it is ready to light a lamp the instant it is turned on, because, as you have seen, electricity will always get back to the dynamo if there is a possible chance, and the lamp opens a path.

The volts pressure used to operate any number of incandescent lamps is altogether very much less than for a number of arc-lights. For example, in the Edison system the pressure (sometimes called "electromotive force") is only about 110 volts, which is very mild and not at all dangerous. This electromotive force would be the same if there were one lamp or ten thousand lighted.

While this Edison current would not hurt any one, you should remember that it is much the better plan not to touch any electric-light wires until you have learned a great deal more on this subject.

We may add that each of the standard incandescent lamps requires only about one-quarter of an ampère of current to make them give a light of 16 candle-power, which is about the light given by a very good gas-jet, and while the electromotive force, or pressure, would only be about 110 volts, whether there were one lamp or ten thousand lighted, there must be sufficient ampères in the wires to give each lamp its proper quantity.

SWITCHES

We have made mention several times of turning on or off one or more lights, and now, perhaps, you would like to know how this is done.

Suppose the electricity was traveling through wires to one or several lamps, it would light up those lamps as long as the wires provided a path to travel in, but if you were to cut out one of them, which is called "breaking the circuit," there would be no road for the electricity to follow, and, consequently, its course would be stopped short and the lamps would go out. You will remember that electricity must have a complete circuit or it can do no work, and in electric lighting it is always a metallic circuit that is used.

Now, the switch is simply a device which is used to break the circuit so that the current cannot pass on. The simplest form of switch is seen in the sketch. (Fig. 26.)

You will see that there is a wire cut in two, and to one piece is attached a metallic piece, A, which turns one way or the other, and when it is turned so as to touch the other part of the wire the circuit is closed and the electricity goes from the lower part of the wire through the metallic piece A to the other part of the wire, thus making a complete circuit or path for the electricity to travel in.

If we turn the piece A away from the upper wire this breaks the circuit and cuts off the path, and, of course, the lamps would go out.

This is the principle of the switch, and, although they are made in thousands of ways, switches all have the same object--namely, the closing and breaking of the circuit, whether it is for one or a hundred lamps.

WIRE ON DYNAMOS

In explaining to you the construction and working of dynamo-machines, we did not state anything about the amounts of wire used in winding the machine.

It is not our intention to say exactly how much is used on any one dynamo, because that is among the things you will have to learn when you come to study the subject of electricity more deeply.

We simply want to have you understand that upon the number of turns of wire on any one machine depends the effect that that amount of wire, carrying electricity, will have upon a certain weight of iron when the armature is revolved a certain number of turns per minute.

A certain number of strands of wire on an armature will only do a certain amount of work at the most, so you will see that a small dynamo will not produce as much electricity as a larger one containing more iron and wire. For high pressure there must be more strands of wire cutting the lines of force more frequently than would be required for low pressure; and, to produce a great many ampères, the armature must be larger and the wire upon it thicker than it would need to be if only a small number of ampères were wanted.

This of itself is a very deep and complicated subject, and many books have been written upon it alone. We shall, therefore, not attempt to go more deeply into it in this little book, but simply content ourselves with giving you the general idea, which will be sufficient until you make a thorough study of the subject.

VIII

ELECTRIC POWER

One of the most convenient uses to which electricity is put is in producing motive power for driving all kinds of machines, from a sewing-machine to a railway train, and we will now try to explain how we can get this kind of work from electricity.

To begin with, you all know that a piece of machinery is usually made to work by revolving a wheel which is part of the machine, either by means of a steam-engine or by water-power, or, as a sewing-machine, by foot-power. Now, when we work a piece of machinery by electricity we do just the same thing by using, instead of the steam-engine or water or foot power, an electric-engine called an "electromotor," which operates in the same way--namely, by turning the wheel of the machine it is applied to.

Foot-power is hard work for the person who is applying the power, and, as you can easily see, one person can make only a very little power by use of the feet. Steam and water power can be used for any large amount of work, but the work must be within a few hundred feet of the engine or the power cannot be used.

If there were a factory using steam-power a block or two away from where you lived, and you had a lathe in your house which you would like to have run by the steam-power in the factory, it would be practically impossible to do this. Now, if the factory were still farther away from your house, it would be still more impossible, and if it were a mile away it would be foolish to dream of taking steam-power from a place so far away.

Suppose, however, that this factory was lighted by electric lights, it would be a very easy matter to take some of the power over to your house. This could be done, even if the factory were miles away, by taking two wires from their electric-light wires and running them into your house to an electromotor connected with your lathe. This electromotor would then run your lathe just as well as if it were belted to a steam-engine.