146. The Primary Coil (§ 137) is made by winding 3 layers of No. 24 insulated copper wire upon the insulated core. One end, 6, is fastened to W (See § 109), and the other end, 5, is held under the screw-head, R. Wind at least two layers of paraffined paper around this coil before winding on the secondary coil.
147. The Secondary Coil (§ 138) is made of No. 30 insulated copper wire, there being 11 or 13 layers, each having about 200 turns. This makes, in all, about 2,500 turns of fine wire. If your winder works properly and the long core is strongly held by the winder-nut, you will have no trouble, although it takes a little time to wind on so many turns. The ends of this coil, 7 and 8, are fastened to Y and Z, which are made like App. 46. It will be found best to wrap a piece of thin paper around the coil after every 3 or 4 layers are wound on. This makes better insulation, and makes the winding easier. Protect the coil by covering it with thick paper. The whole coil, when completed, is about 1 in. in diameter.
148. The Automatic Interrupter (Figs. 78, 78-A, 78-B) consists of several parts. B, E, C is a piece of thin tin, all in one piece. The part, B, is 1/4 in. wide and 1-3/4 in. long. Its exact height above the base will depend upon the diameter of your coil. For the coil here described, 1 in. in diameter, the top edge of B is 5/8 in. above the base. See Fig. 78-B for shape of B, E, C before bending it, and for its dimensions. Around the end of B are tightly wound several turns of tin, making the armature or hammer, A, which should not be allowed to strike against the head of the bolt on account of residual magnetism. (See text-book.) A piece of thick paper pasted on the head for A to strike upon is best. A will probably not get near enough to the bolt to strike it, but this will depend upon how you arrange the parts.
D is a wooden piece, 1 in. high, 1 in. wide, and 3/8 or 1/2 in. thick; it is nailed to the base. Through its center is a hole for the screw-eye, S I, which is the regulating-screw. F is a piece of copper, brass, or tin, 5/8 × 1-3/4 in. It is held to the base by the screw, S, and is bent so that it presses tightly against S I. Through F is a screw, R, to hold one end of the primary coil.