cuit must be opened and closed at but one place at a
time. As soon as you have finished, your switch must be closed. He will
open his and proceed. When you have both finished, both switches must be
closed. If your friend left his switch open, you could not call him over
the line, as no current could pass into his sounder.

195. Batteries. As the circuit has to be left closed for hours and
perhaps days at a time, so that either operator can call the other, a
closed-circuit battery is necessary. (See App. 9.) A dry cell,
Leclanche, or other open-circuit cell would not be at all suitable for a
telegraph line, as it would soon polarize. Large Daniel cells, which are
2-fluid cells like App. 7, or gravity cells (App. 9) are the best for
your line.

APPARATUS 120.

_196. Telegraph Sounder._ Fig. 94. The wood-work consists of 2 parts;
the base, B, is 6 x 4 x 3/4 in., and the back, A, is 6 x 5 x 1/2 in. A
is nailed or screwed to B.

The Magnet, M, is fully described in App. 85. M is held firmly to A by
cord or wire, which should pass around it near the poles and at the
curved part. The wire should pass through small holes in A, and be tied
at the back. Wire nails driven into A at the sides of M will keep it
from moving about. The wires from the magnet coils are led to two spring
binding-posts, X and Y.

[Illustration: Fig. 94.]

197. The Armature, C, is made of a narrow piece of thin iron, about
5-1/2 x 1/4 x 1/8 in. It may be made by bending up 3 or 4 thicknesses of
tin into that shape. This is the part which will be attracted by M, when
the current passes, and which will make the clicks by which the message
can be read. (See telegraph alphabet.) There are many ways by which C
can be held near M. The figure shows how it can be done entirely with
1-in. wire nails. At the right end of C two nails are driven into A
above and below C. They are just far enough apart to allow the left end
of C to be raised and lowered without binding; in other words, these
nails make a pivot for C to swing upon, and they help to support it at
the same time. The left end of C must not quite touch the poles of M
when the current passes, because the residual magnetism would keep C
from dropping back into place. To adjust the armature, pass the current
through M, hold C so that it will not quite touch the poles, then drive
in the upper nail, 2. Put another nail, 1, below C, so that M will not
have to lift C more than 1/8 or 3/16 in. Try the nail