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.) [Illustration: 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.