04, this arrangement will be
found good. The base is 5 x 4 x 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.

[Illustration: Fig. 76.]

The Connections. We shall suppose that you have the interrupter of App.
104, Fig. 81. The ends of the primary coil (Sec. 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.

APPARATUS 99.

_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
x 4 x 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.

[Illustration: Fig. 77.]

APPARATUS 100.

_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 Sec. 119.) The core
should be insulated with paraf