_143. Induction Coil._ Fig. 76. In case you wish to make the interrupter as a separate piece of apparatus, as App. 104, this arrangement will be found good. The base is 5 × 4 × 7/8 in. The coil is explained in App. 96, and the methods of holding it to the base are given in App. 97. The binding-posts are like App. 46.
The Connections. We shall suppose that you have the interrupter of App. 104, Fig. 81. The ends of the primary coil (§ 137) are fastened under the screws of X and W, and those of the secondary coil to Y and Z. Connect one battery wire with X and the other battery wire to the interrupter at S, Fig. 81. Fasten the end of a stout wire to W, and leave the other end free to scrape along on the nails, Q, of the interrupter. This will then open and close the primary circuit. The handles (App. 101) are connected with Y and Z, as explained in App. 97. Use the battery of App. 3 or 4.
_144. Induction Coil._ Fig. 77. If you wish to fasten your coil in an upright position the apparatus will look like Fig. 77. The base may be 5 × 4 × 7/8 in. The binding-posts are like App. 46. The coil is made as explained in App. 96; but to have all the ends of the coils come out at the bottom, as shown, an even number of layers of wire will be necessary. It will be just as well to have an odd number of layers as before, and to bring the wire ends down the side of the coil. The coil is fastened to the base with screws, S, passing through a tin strip, T, which has a hole punched for the bolt. T is squeezed between the regular nut on the bolt and an extra one on the underside of it. See Fig. 61 for suggestion of another method of holding bolts upright. The connections should be made with an outside interrupter, battery, and handles, as explained in App. 98.
_145. Induction Coil._ Fig. 78, 78-A, 78-B. In case you wish to make a larger coil than those already described, the following will be found practical. It is made in the same general way as before, an automatic interrupter, however, being added.
The Core is a machine-bolt, 4-1/2 in. long and 5/16 in. in diameter. You may use a carriage-bolt of the same dimensions, if you file away the square shoulder at the head end, so that it will be the same size as the body of the bolt. Paste a piece of thick paper upon the head, so that A will strike the paper instead of the iron. The Washers should be made around a spool that is fully 1 in. in diameter. (See § 119.) The core should be insulated with paraffine paper before winding on the primary coil. (See App. 88.) The washers are 3-7/8 in. apart, inside. The winding of the coils should be done with App. 93, or some other winder. The winder-nut, W N, Fig. 70, must hold the long core perfectly tight, to avoid wobbling. The base is 8 × 5 × 7/8 in. The different parts are placed as shown. The coil is fastened to the base as in App. 97. For binding-posts see App. 46.