DIFFERENT KINDS OF DYNAMO.--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. DIRECT-CURRENT DYNAMO.--We shall now make the simplest form of dynamo, using for this purpose a pair of permanent magnets. [Illustration: _Fig. 22._ DYNAMO FIELD AND POLE PIECE] SIMPLE MAGNET CONSTRUCTION.--A simple way to make a pair of magnets for this purpose is shown in Fig. 22. A piece of round 3/4-inch steel core (A), 5-1/2 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. HOW TO WIND.--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'). THE DYNAMO FIELDS.--Two of these spools are so made and they are called the _fields_ of the dynamo. We will next prepare an iron bar (I), 5 inches long and 1/2 inch thick and 1-1/2 inches wide, then bore two holes through it so the distance measures 3 inches from center to center. These holes are to be threaded for the 3/4-inch cores (A). This bar holds together the upper ends of the cores, as shown in Fig. 23. [Illustration: _Fig. 23._ BASE AND FIELDS ASSEMBLED]