(Sec. 119) that is fully 1 in.
in diameter.

137. The Inside or Primary Coil could be wound directly upon the bolt;
but it is much better to cover the bolt with one or two thicknesses of
paraffined paper, I (Index), as shown. A pinhole, H, in the washer is
for the inside end (see Sec. 123) of the primary coil, and the hole,
J, is for the outside end of it.

The primary coil should be made of 3 layers of wire, which should be
coarser than that used for the secondary coil. For our purposes it is
best not to use a wire coarser than No. 20, and not finer than No. 24.

Use No. 24 insulated copper wire if you are going to connect ordinary
batteries with it. A bichromate cell (App. 4) is best. Put about 6 in.
(see Sec. 109) of wire through H, and with App. 93 wind on 3 layers of
say No. 24 wire. There being an odd number of layers, the winding will
stop at the head end of the bolt, where a half hitch (see Sec. 110)
should be taken before passing the wire through the hole, J. Cut the
wire 6 in. from the hole. Write down the number of turns of wire to
each layer and the total number of turns. You now have a 3-layer coil,
and a current passed through this will magnetize the bolt; you
have--so far--merely an electro-magnet. Cover the primary coil with 2
layers of paraffined paper, K (Fig. 74), and put some paraffine
between the edges of K and the washers, so that the wire of the
secondary coil cannot possibly come in contact with that already wound
on.

138. The Secondary Coil should be made of a large number of turns of
fine wire. Do not use anything coarser than No. 30. This is a good size,
as finer wire is very easily broken by unskilled hands. For the size of
bolt mentioned put on 13 layers. There will be about 100 turns to each
layer, making a total of about 1,300 turns of No. 30 wire. Write down
the total number of turns in your coil. To start the secondary coil,
make a pinhole, L, just outside of the insulation, K, of the primary
coil. Put 6 in. of wire through this, wind the end around the nut (App.
93, Fig. 70), and wind on as evenly as possible 13 layers. If the layers
become rough, it is well to put a band of paper around after each 3 or
4. When you have finished take a half hitch (Sec. 110), and leave a 6-in.
length free. Cover the secondary coil with strong paper. This coil may
be used on any of the forms of shockers given.

APPARATUS 97.

139. Induction Coil. Fig. 75. The base is made of a piece of board, 7