s in different
positions until C quickly rises and falls when the circuit is closed and
opened. A nail, 3, driven in front of C, will keep its right end in
place. No springs are needed, as gravity acts upon C instantly, bringing
it to the lowest position as soon as the current ceases to flow.

198. The Battery will depend upon how much you want to use the sounder.
If just to show the principle of it, almost any cell of medium strength
will do, like that of App. 3, 4 or 5. A dry battery will do, but if you
use the sounder much, an open-circuit battery will soon use itself up.
Where much work is needed of the battery use App. 9.

[Illustration: Fig. 95.]

The Key like App. 119 is best. Push-buttons are handy where used only
for experiments, and not for the actual sending of messages.

APPARATUS 121.

_199. Telegraph Sounder._ Fig. 95. This makes a simple and efficient
sounder for short lines. The base, B, is 7 x 4-1/2 x 7/8 in. The back,
A, is 7 x 4-1/2 x 1/2 in.; it is nailed to B. The piece D is 4 x 3/4 x
3/4 in.; it is nailed to A. C is a wooden piece 1-1/2 x 3/4 x 3/4 in.;
it is nailed to A, and in its top is a screw, E, which is used as a
regulating-screw to keep the armature, L, from touching the poles.

200. The Armature, L, is explained as App. 77. The two thicknesses of
tin at F must not be too thick, or it will take too much battery power
to work the sounder. If you find that it is too stiff to bend down, when
the current is on, try the arrangement of App. 122, which is easier to
make and regulate. The whole point depends upon the tin you have. The
end of L must tap against E. A hole is punched in the part F, and a
screw, G, holds it to D. L should rest about 1/8 in. above the poles and
gently press against a screw or nail, V.

201. The Magnets are like App. 89. They are made as in App. 88, and held
down like App. 90. These should be placed very near the back, A, so that
the armature will be over them. If your yoke is not too wide the coils
may rest against A. Y and Z are binding-posts like App. 46.

202. Connections. Join the coils as explained in Sec. 125 and see Sec.
115. Instead of a third or middle binding-post, as in Fig. 66, hold the
two inside ends between a screw-head and a copper bur. The method of
joining the wires for a line with two outfits, is shown in App. 124. If
you have but one key, sounder, and battery, simply join the line wire
to the return wire there shown. A gravity cell is best.