t still coming into W from the
battery; the current will pass along B, out through Z, into the
galvanometer, back to Y, through D, and back to the battery. It is
evident, then, that the current can be made to pass out of Y or Z to the
galvanometer at will by pressing down A or B.

APPARATUS 130.

[Illustration: Fig. 104.]

_228. Current Reverser._ Fig. 104. The wooden base is 7 x 5 x 7/8 in. To
this are fastened two brass or tin straps, C and D, 5 x 1/2 in. They are
fastened at the front ends by screws, S, while the binding-posts, Y and
Z, hold the other ends solid. X and W are two screw-eye binding-posts
(App. 45). The small square piece of wood, T, is 3 x 3 x 1/2 in. Through
the corners of T, and in positions so that they will be directly over C
and D, are put four screw binding-posts, 1, 2, 3, 4 (App. 41). The
screws, however, pass entirely through T, and stick out about 1/4 in. on
the underside of it. The wire, A, connects W, 1 and 4, while the wire,
B, connects X, 2 and 3. A and B must not touch each other where they
cross on the top of T. N is a wire nail that serves as a handle. If we
were to place T, holding the four corner screws, upon the straps, C and
D, it is evident that all the screws would touch the straps, if they
were properly adjusted. We must fix things so that two only can touch
the straps at a time. Put a screw, Q, through the center of T, from the
bottom, so that it will stick out of the bottom more than the screws, 1,
2, etc. The screws, 2 and 4, will be lifted from C and D when the
handle, N, is pressed down. By raising N, the top, T, can be made to
rock up and down upon Q as a pivot. By lifting N far enough, 2 and 4
will be pressed against C and D, while 1 and 3 will be raised. A spring,
R, is shown joined to T and to the base. This will hold the screws, 2
and 4, down upon C and D, unless N is pressed down.

229. Operation. We shall first suppose that the spring, R, is holding 2
and 4 in contact with C and D; 1 and 3 will, of course, be held up in
the air. Imagine that we have a galvanometer connected with Y and Z. If
the battery current enters at W, it will pass along A to 4, before it
can find a chance to escape. It will pass through 4 into D, and into the
galvanometer by way of Z, then back by way of Y, up 2, and out to the
battery from X. If we now press the handle, N, down, the current will
pass from W to 1, down 1 through C and Y to the galvanometer. It will
return to the battery by