x
5 x 7/8 in. The locations of the different parts are shown in the
figure. The coil is explained in detail in App. 96. It is fastened to
the base by a thin copper strip, 4, which is bent over the coil and held
down by screws, 3. If you haven't any copper you can use a narrow strip
of tin. Do not use a wide piece of tin or iron. The coil may be held
down firmly by strong twine placed around each end of it. The twine
should pass through holes in the base, and be tied on the underside of
the base. The binding-posts are like App. 46.

140. The Current Interrupter consists of a tin or copper strip, R, 6 in.
long and 1/2 or 3/4 in. wide. At one end of R is a screw, S, which is
used as a binding-post for the outside end, B, of the primary coil. (See
Sec. 137.) Along the center line of the strip, R, are driven 1-in. wire
nails, Q. These are placed 1/4 in. apart, and they should go into the
wood enough only to make them solid. (See Fig. 81.) Do not drive them in
so far that they will split the base. A stout wire, P, fastened at one
end only completes the interrupter.

[Illustration: Fig. 75.]

141. The Connections. The binding-posts, W and X, should be connected
with the wires leading from a battery. Use the bichromate batteries of
App. 3 or 4. A dry battery will do. If the current enters at X, it will
pass around the primary coil (Sec. 137) and out through B into R. It can
go no farther until the free end of P is made to touch R, or one of the
nails, Q, when the circuit will be closed. The current will fly around
and around through the battery, primary coil, and interrupter as long as
the end of P touches a nail. The battery current does not get into the
secondary coil at all. You can see, then, that the primary circuit, that
is, the one passing through the coarse wire, will be rapidly opened and
closed by bumping the free end of P along upon the row of nails.

The wires, C and D, coming from the secondary coil (Sec. 138) are in
connection with Y and Z, to which are connected the wires leading from
the handles (App. 101) held by the person receiving the shock.

142. To use the coil, arrange as explained. Let your friend hold the
handles (App. 101) while you scrape the end of P back and forth along
the row of nails. For those who cannot stand much of a shock, use a
regulator (App. 103).

APPARATUS 98.

_143. Induction Coil._ Fig. 76. In case you wish to make the interrupter
as a separate piece of apparatus, as App. 1