in one
direction, and again made at once with the current reversed. The use of
commutators on motors and dynamos should be understood. The reversers
herein shown are, of course, not at all like those used on motors.
Current reversers are used in connection with the needle-telegraph and
many other instruments.

[Illustration: Fig. 103.]

APPARATUS 129.

_225. Current Reverser._ Fig. 103. The base is 5 x 4 x 7/8 in. To this
are fastened four metal straps, A, B, C, and D. These may be made of
brass, aluminum, or even of tin. If made of tin, use one thickness of
metal for C and D, and two thicknesses for A and B. Each strap has two
1/8 in. holes punched in it, their positions being shown by the
screw-heads and screw-eye binding-posts.

Construction. C is 3-3/4 x 1/2 in. Fasten this to the base first. At the
left end is a small screw, while the right end is held down by the
binding-post, W. The keys, A and B, should have quite a little spring to
them. These are cut 5 x 3/4 in. The front end of each is bent over a
little (see the key App. 118, Fig. 92) so that they may be more easily
grasped. The length after bending will be less than 5 in. The front ends
should be raised from the base (Fig. 92) so that they will not touch C,
unless pressed down. The 1/8 in. holes in the end of A are about 3/4 in.
apart, one being used for a screw to hold it to the base, and the other
for the binding-post, Y. The strap, D, is 3-3/4 x 1/2 in. It is fastened
at one end by a screw, and at the other end by X. D is bent about 3/4
in. from each end, so that its middle part stands above the base about
1/4 in. The straps, A and B, press up against D, unless they are held
down with the hand.

226. Connections. W and X are joined to the poles of the battery to be
used. Y and Z are joined to the apparatus in which the current must be
passed in one direction, and then in the opposite direction. A tangent
galvanometer, or a needle-telegraph instrument, for example, may be
connected with Y and Z.

227. Operation. Suppose that the battery current enters at W. As long as
both keys are raised, the current can go no farther. Now, imagine that
we press A down solidly upon C, the current will pass along A, which
does not now touch D, out through Y into the galvanometer, back to Z,
into D, and to the battery again; that is, the current will enter the
galvanometer from Y. Now, suppose that we let A spring up against D
again, and press B down, the curren