used instead of the spools; they should be left sticking out from C
about 1 in. Around the spools or screws, fasten a pasteboard band, G, on
which to wind the wire. G may be about 1 in. wide; it should be kept in
the circular form by sewing the ends together where they lap. (Read
directions in App. 116.)

191. The Coils on this model are 4 in number. (See App. 116 for the
method of winding.) The first coil is made of coarse wire, No. 18, its
ends being joined to the binding-posts, V and W. The second coil has 5
turns of No. 24 insulated copper wire, its ends being joined to W and X.
The third coil has 10 turns of the same size wire, No. 24, and is joined
to X and Y. The fourth coil has 20 turns of the same joined to Y and Z.
If you want to use the galvanometer for quite weak currents, it would
be well to make a fifth coil of 20 turns of No. 30 wire, and join it
with Z and a new binding-post. The ends of the coils are run through
small screw-eyes before passing to X, Y, etc. This is not necessary, it
merely keeps them in place.

The Binding-Posts are like App. 43. Any other desired style may be used,
those of App. 46 being preferred.

The Hole, H, is 5 in. in diameter. It should be cut out about 1/2 in.
below the center of the circles to allow for D, and for the pin-point
which supports the magnetic needle, the poles of which should be in the
line passing through the center of the coils. The method of cutting the
hole, H, through C, will depend upon the tools at your service.

D is the front edge of an adjustable table, like that explained.
(Index.) It is 4-1/4 in. wide. It supports the magnetic needle which is
inside of E.

E is the outside of a glass-covered compass. (See App. 67 for details.)
The needle should not be over 1 in. long.

CHAPTER XIV.

TELEGRAPH KEYS AND SOUNDERS.

APPARATUS 118.

[Illustration: Fig. 92.]

_192. Telegraph Keys._ Fig. 92. Telegraph keys are merely pieces of
apparatus by which the circuit can be conveniently and rapidly opened or
closed at the will of the operator. An ordinary push-button may be used
to turn off and on the current, but it is not so convenient as a "key."
Fig. 92 shows a side view of a simple key. C is a metal strip about 3/4
in. wide and 4 or 5 in. long. At the left end it is fastened to the base
with a screw, A. Another screw, X, serves as one binding-post. Y is
another screw binding-post. W is a short wire, used to regulate the
amount of spring to the