2 in. square and 7/8 in. thick. A set-screw, S,
binds it to the dowel-axle, D, which is made to turn by one of the forms
of cranks given, and which is held in one of the frameworks. Windings
like that shown in App. 112, Fig. 85, can easily be done with this, the
upright part, with the two spools, being screwed right to A of Fig. 72.

[Illustration: Fig. 72.]

CHAPTER XI.

INDUCTION COILS AND THEIR ATTACHMENTS.

_134. Induction Coils_, or shocking coils, are rather expensive to buy,
and altogether too complicated for boys to make by the methods usually
given in books. The method here given is simple, the materials are
cheap, and if you make them according to directions, you will have an
apparatus that will, be able to make your friends dance to a rather
lively tune. The amount of shock can be regulated perfectly (App. 103).

Winding. Full instructions have been given for making bolt magnets (App.
88). The winding of our induction coils is done in the same way by the
same winder as the bolt magnets (App. 93), or by hand. You will find it
a very tiresome and troublesome job, however, to wind on 12 or 15
hundred turns of fine wire by hand. Make a winder.

Several different forms of induction coils are shown. The coil is the
most important feature, however, and we shall consider that separately.
When you understand the construction of one coil, you can readily apply
this to the different forms. Some form of contact breaker, or current
interrupter, is needed also. These will be treated by themselves. The
connections will be discussed under each form of apparatus.

APPARATUS 96.

_135. Induction Coil; Construction of Coil Proper._ Figs. 73, 74. An
induction coil is a peculiar and wonderful apparatus. There are at least
two coils to each one. These are both wound upon the same core. They
are made of different sizes of wire, are wound separately, and the
strangest thing of all is, that these two coils are not connected with
each other in any way. If they were not thoroughly insulated from each
other, the coil would be of no value. (Study induction.) The winding of
the two coils is done as explained in App. 88.

[Illustration: Fig. 73.]

[Illustration: Fig. 74.]

136. The Core is made of a 5/16 machine bolt, 2-1/2 in. long. Leave but
2 or 3 threads at the end, just enough to fasten it solidly to the
winder (App. 93). The washers should be about 1-5/8 in. apart inside,
and they should be made around a spool