fine paper before winding on the primary
coil. (See App. 88.) The washers are 3-7/8 in. apart, inside. The
winding of the coils should be done with App. 93, or some other winder.
The winder-nut, W N, Fig. 70, must hold the long core perfectly tight,
to avoid wobbling. The base is 8 x 5 x 7/8 in. The different parts are
placed as shown. The coil is fastened to the base as in App. 97. For
binding-posts see App. 46.

[Illustration: Fig. 78.]

146. The Primary Coil (Sec. 137) is made by winding 3 layers of No. 24
insulated copper wire upon the insulated core. One end, 6, is fastened
to W (See Sec. 109), and the other end, 5, is held under the screw-head,
R. Wind at least two layers of paraffined paper around this coil before
winding on the secondary coil.

147. The Secondary Coil (Sec. 138) is made of No. 30 insulated copper
wire, there being 11 or 13 layers, each having about 200 turns. This
makes, in all, about 2,500 turns of fine wire. If your winder works
properly and the long core is strongly held by the winder-nut, you will
have no trouble, although it takes a little time to wind on so many
turns. The ends of this coil, 7 and 8, are fastened to Y and Z, which
are made like App. 46. It will be found best to wrap a piece of thin
paper around the coil after every 3 or 4 layers are wound on. This
makes better insulation, and makes the winding easier. Protect the coil
by covering it with thick paper. The whole coil, when completed, is
about 1 in. in diameter.

[Illustration: Fig. 78-A.]

[Illustration: Fig. 78-B.]

148. The Automatic Interrupter (Figs. 78, 78-A, 78-B) consists of
several parts. B, E, C is a piece of thin tin, all in one piece. The
part, B, is 1/4 in. wide and 1-3/4 in. long. Its exact height above the
base will depend upon the diameter of your coil. For the coil here
described, 1 in. in diameter, the top edge of B is 5/8 in. above the
base. See Fig. 78-B for shape of B, E, C before bending it, and for its
dimensions. Around the end of B are tightly wound several turns of tin,
making the armature or hammer, A, which should not be allowed to strike
against the head of the bolt on account of residual magnetism. (See
text-book.) A piece of thick paper pasted on the head for A to strike
upon is best. A will probably not get near enough to the bolt to strike
it, but this will depend upon how you arrange the parts.

D is a wooden piece, 1 in. high, 1 in. wide, and 3/8 or 1/2 in. thick;
it is nailed to th