The third method of generating electricity is by electro-magnets. The value and use of induction will now be seen, and you will be enabled to utilize the lesson concerning magnetic action referred to in the previous chapter.
You will remember that every piece of metal which is within the path of an electric current has a space all about its surface from end to end which is electrified. This electrified field extends out a certain distance from the metal, and is supposed to maintain a movement around it. If, now, another piece of metal is brought within range of this electric or magnetic zone and moved across it, so as to cut through this field, a current will be generated thereby, or rather added to the current already exerted, so that if we start with a feeble current, it can be increased by rapidly "cutting the lines of force," as it is called.
While there are many kinds of dynamo, they all, without exception, are constructed in accordance with this principle. There are also many varieties of current. For instance, a dynamo may be made to produce a high voltage and a low amperage; another with high amperage and low voltage; another which gives a direct current for lighting, heating, power, and electroplating; still another which generates an alternating current for high tension power, or transmission, arc-lighting, etc., all of which will be explained hereafter.
In this place, however, a full description of a direct-current dynamo will explain the principle involved in all dynamos - that to generate a current of electricity makes it necessary for us to move a field of force, like an armature, rapidly and continuously through another field of force, like a magnetic field.
We shall now make the simplest form of dynamo, using for this purpose a pair of permanent magnetsFig. 22. Dynamo Field and Pole Piece
A simple way to make a pair of magnets for this purpose is shown in Fig. 22. A piece of round ¾-inch steel core (A), 5½ inches long, is threaded at both ends to receive at one end a nut (B), which is screwed on a sufficient distance so that the end of the core (A) projects a half inch beyond the nut. The other end of the steel core has a pole piece of iron (C) 2" × 2" × 4", with a hole midway between the ends, threaded entirely through, and provided along one side with a concave channel, within which the armature is to turn. Now, before the pole piece (C) is put on, we will slip on a disc (E), made of hard rubber, then a thin rubber tube (F), and finally a rubber disc (G), so as to provide a positive insulation for the wire coil which is wound on the bobbin thus made.
In practice, and as you go further along in this work, you will learn the value, first, of winding one layer of insulated wire on the spool, coating it with shellac, and then putting on the next layer, and so on; when completely wound, the two wire terminals may be brought out at one end; but for our present purpose, and to render the explanation clearer, the wire terminals are at the opposite ends of the spool (H, H').