Fig. 116.
253. The shaft or axle, 8, is made of a "sink-bolt" that is 3 in. long and ¼ in. in diameter. These sink-bolts are threaded over their entire length, and are furnished with two nuts, 9 and 11, Fig. 117. File or grind the end of 8 to a point, so that it will turn easily in a dent made for it in the yoke, D, or in a dent made in another piece of tin fastened over the yoke. The shaft is held in a vertical position by the arm, C.
254. The Arm, C, is made of 2 or 3 thicknesses of tin. It is 3 × ¾ in.; it has in one end a hole for the shaft to revolve in easily, and in its other end a slot is cut. A screw-eye and bur are used to hold C to the upright, U. By this means the shaft can be moved and regulated as to position.
Fig. 117.
255. The Commutator, 9, (Fig. 117), is made of one of the nuts furnished with the shaft. Two of its corners are filed or ground off, so that it has the shape shown at the right, in Fig. 117. The copper wire, 10, rubs against 9, as the pointed part of it comes around. 10 is really a "brush," and carries the current into 9 at the right time.
256. Connections. Join the two inside ends ([§ 123]) of the coils to 4. The outside end of 2 is joined to X; the outside end, 7, of the other coil, 6, is carried up under or around the screw-eye, S I, and then its bare end reaches out and gently scrapes against the top of the shaft, 8. The wire, 10, leads from Y to the back of the base, where it is carried up to a screw, 12, which holds it to U. Its bare end reaches out to gently scrape against the commutator, 9, when it swings around. This wire, 10, should not press against 9 during the entire revolution.
257. Adjustment. Suppose the current enters at X. When the "brush," 10, presses against the commutator, 9, the current passes through X, 1, 2, 3, 4, 5, 6, 7, down 8 to 9, and out through 10 to Y. (The current, of course, goes down into D and into the bolt-cores also; but it can go no farther, if the coils are properly insulated, and A is not allowed to touch the cores. It is better to have the end of the shaft rest upon a piece of glass, having a slight depression made with a file, or in a dent made in tin which rests upon wood, the tin having no connection with D.) If 10 should continue to press against 9, the current would continue to pass, and A would be held firmly in place, directly over 2 and 6, and, of course, the shaft could not revolve. If, however, the brush leaves 9 (See plan of 9 at side of Fig. 117), just as A gets over the coils, or an instant before it gets there, the weight of A will carry it beyond the coils. No current should pass again, until A is at least at right angles to a line drawn through the center of the coils. If the current again passes, the ends of A will be attracted by the bolt-cores.