half inch thick, and shellac thoroughly
on all sides. Then prepare sheets of tinfoil (B), six by eight inches in
size, and also sheets of paraffined paper (C), seven by nine inches in
dimensions. Also cut out from the waste pieces of tinfoil strips (D),
one inch by two inches. To build up the condenser, lay down a sheet of
paraffined paper (C), then a sheet of tinfoil (B), and before putting
on the next sheet of paraffined paper lay down one of the small strips
(D) of tinfoil, as shown in the illustration, so that its end projects
over one end of the board A; then on the second sheet of paraffine paper
lay another sheet of tinfoil, and on this, at the opposite end, place
one of the small strips (D), and so on, using from 50 to 100 of the
tinfoil sheets. When the last paraffine sheet is laid on, the other
board is placed on top, and the whole bound together, either by wrapping
cords around the same or by clamping them together with bolts.

[Illustration: _Fig. 72._ CONDENSER]

You may now make a hole through the projecting ends of the strips, and
you will have two sets of tinfoil sheets, alternately connected together
at opposite ends of the condenser.

Care should be exercised to leave the paraffine sheets perfect or
without holes. You can make these sheets yourself by soaking them in
melted paraffine wax.

CONNECTING UP A CONDENSER.--When completed, one end of the condenser is
connected up with one terminal of the secondary coil, and the other end
of the condenser with the other secondary terminal.

[Illustration: _Fig. 73._ HIGH-TENSION CIRCUIT]

In Fig. 73 a high-tension circuit is shown. Two coils, side by side, are
always used to show an induction coil, and a condenser is generally
shown, as illustrated, by means of a pair of forks, one resting within
the other.

THE INTERRUPTER.--One other piece of mechanism is necessary, and that is
an _Interrupter_, for the purpose of getting the effect of the
pulsations given out by the secondary coil.

A simple current interrupter is made as follows: Prepare a wooden base
(A), one inch thick, six inches wide, and twelve inches long. Upon this
mount a toothed wheel (B), six inches in diameter, of thin sheet metal,
or a brass gear wheel will answer the purpose. The standard (C), which
supports the wheel, may be of metal bent up to form two posts, between
which the crankshaft (D) is journaled. The base of the posts has an
extension plate (E), with a binding post for a w