In other words, the current should pass a little less than one-half of the time, and this is divided into two parts. Suppose you start A with your finger; the current should be shut off automatically just before the center of A gets over the center of the bolt-cores. A makes ¼ of a revolution without current, and just after it gets beyond this, the current passes for nearly ¼ of a revolution, which brings the ends over the poles again. The next ¼ of a turn it has no current, because the flat side of 9 is opposite the brush, 10, as during the first ¼. The last ¼ the current passes again. The exact position of the commutator will depend upon the way you arrange the brush. The positions of 9 and 10 can be found by trial, so that the circuit will be promptly opened and closed at the proper moment. Start the motor by turning the armature.
258. Batteries. The amount of power needed will depend upon how well you make the motor. One cell of [App. 3] or [4] will run a well made one, but it is better to use 2 cells. Join the wires to X and Y.
Fig. 118.
259. Armature for Motors. Fig. 118 shows another form of armature that may be used for small motors like [App. 144]; in fact, you may find that this form is easier to make than that of [App. 144]. M is a 5⁄16 machine screw, 1½ in. long, 9 being the nut furnished with it. 9 is filed as explained in [§ 255], and forms the commutator. C is the arm ([§ 254]). A is the armature ([§ 252]). A is held firmly in place between the spool, E, and 9. S is a set-screw which passes through E, and holds the piece of ¼ in. dowel, F, in place. N is a needle-point fastened in the end of F. N revolves in a dent made in a piece of tin, H, which rests upon a wooden strip, G. G is cut away on its underside, so that it will straddle the yoke, D, Fig. 117; it is nailed to the base. This is given as a suggestion. By making F a little longer, N can turn in a dent made in the yoke, below G.
260. Adjustments. M, being 5⁄16 in. in diameter, will screw solidly into the hole in E. Place 9 upon it first, then A, and screw it about ½ way into E. 9 will serve as a lock-nut by turning it so that it will pinch A and hold it firmly against the top of E. F should reach half way into E. Put N in place after you have H and G arranged. You can then cut the upper end of F at such a place that it will bring A about ⅛ in. from the top of the magnet-cores. Paper wrapped around F will make a good fit in E. The current should enter M and leave 9, as fully explained in [App. 144]. ([See § 257]).
261. Electric Motor. Fig. 119, 120, 121, 122. Fig. 119 shows a front view, and Fig. 120 a side view of the whole motor. Fig. 121 shows the part that revolves, and includes the shaft, armature and commutator. Fig. 122 shows a section of the commutator. All the dimensions are taken from a model. You can modify the size to suit.