bearing for F to rock up and down upon.
The hole for N is 2 in. above B. It must not be too loose in the holes,
or F will rock sidewise, and allow A to touch one of the magnets. The
upright, E, is 2-3/4 x 3/4 x 3/4 in., and is screwed or nailed to B from
below. A screw, G, is put into the side of E near the top. This screw
has the underside of the head filed flat, and against this the screw, L,
taps when the armature is attracted. The arm, F, which carries the
armature, A, is 4-1/2 x 1/2 x 1/2 in., and is pivoted by means of N,
which passes through it and the uprights C. F must swing up and down
freely. The hole for N, in this model, is 1-3/4 in. from the armature
end.

207. The armature is fastened to F by a screw, S. A copper bur is put
under the head of S to aid in keeping A from rocking sidewise. Through
F, and about half way between C and L, is put a screw, I, the lower end
of which taps against the head of a screw, H, which is put into D. By
unscrewing H a little, F will be raised, and A will be brought nearer
the poles of M. The rubber-band, J, is placed over the head of I, and
has tied to it a thread, O, which in turn is tied to a screw-eye, K. K
screws into the end of B, and by turning it one way or the other, the
tension, or pull, on J may be increased or diminished. There must be
enough spring in J to pull A up after the current ceases; it must not
pull so much that the magnet cannot draw A down hard enough to make a
good click between L and G.

The Magnet, M, is explained in App. 89, and the construction of one bolt
magnet is given in detail in App. 88. In this particular sounder the
bolts are 2 in. long under the heads, thus bringing the tops of the
bolt-heads about 2-1/4 in. above B. M is held to the base by a band of
tin, T. The yoke may be screwed to B, as suggested in App. 90. This is
the better plan.

208. Adjustment. You will find, although you make all of the parts with
the dimensions given, that you will have to try, and change, and adjust
before everything will work perfectly. A must not be allowed to touch
the poles of M when it is pulled down, on account of the residual
magnetism, which would keep it pulled down. Adjust this with F. The
armature must not be pulled too far up from the poles of M by the
tension in J; adjust this with I and H. If your battery is weak, the
pull of J must be small, just enough to raise A.

The Battery. It is supposed, if you make an instrument like this, that
yo