Electricity for Boys. James Slough Zerbe
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
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Simple Magnet Construction.—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 ofp. 40 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 ½ inch thick and 1½ 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 ¾-inch cores (A). This bar holds together the upper ends of the cores, as shown in Fig. 23
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We then prepare a base (J) of any hard wood, 2 inches thick, 8 inches long and 8 inches wide,p. 42 and bore two ¾-inch holes 3 inches apart on a middle line, to receive a pair of ¾-inch cap screws (K), which pass upwardly through the holes in the base and screw into the pole pieces (C). A wooden bar (L), 1½" × 1½", 8 inches long, is placed under each pole piece, which is also provided with holes for the cap screws (K). The lower side of the base (J) should be countersunk, as at M, so the head of the nut will not project. The fields of the dynamo are now secured in position to the base.
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| Figs. 24–25. Details of the Armature | |
The Armature.—A bar of iron (Fig. 24), 1" × 1" and 2¼ inches long, is next provided. Through this bar (1) are then bored two 5/16-inch holes 1¾ inches apart, and on the opposite sides of this bar are two half-rounded plates of iron (3) (Fig. 25).
Armature Winding.—Each plate is ½ inch thick, 1¾ inches wide and 4 inches long, each plate having holes (4) to coincide with the holes (2) of the bar (1), so that when the two plates are applied top. 43 opposite sides of the bar, and riveted together, a cylindrical member is formed, with two channels running longitudinally, and transversely at the ends; and in these channels the insulated wires are wound from end to end around the central block (1).
Mounting the Armature.—It is now necessary to provide a means for revolving this armature. To this end a brass disc (5, Fig. 26) is made, 2 inches in diameter,⅛ inch thick. Centrally, at one side, is a projecting stem (6) of round brass, which projects out 2 inches, and the outer end is turned down, as at 7, to form a small bearing surface.
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| Figs. 26–27. Armature Mountings | |
The other end of the armature has a similar disc (8), with a central stem (9), 1½ inches long, turned down to ¼-inch diameter up to within ¼ inch of the disc (7), so as to form a shoulder
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The Commutator.—In Fig. 27 is shown, at 10, a wooden cylinder, 1 inch long and 1¼ inches in diameter, with a hole (11) bored through axially, so that it will fit tightly on the stem (6) of the disc (5). On this wooden cylinder is driven a brass or copper tube (12), which has holes (13) opposite each other. Screws are used to hold the tube to the wooden cylinder, and after they are properly secured together, the tube (12) is cut by a saw, as at 14, so as to form two independent tubular surfaces
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