COMMUTATORS AND CURRENT REVERSERS.
_224. Commutators and Current Reversers_ are useful in some experiments,
as, for example, those with tangent galvanometers (App. 116, 117), in
which readings are made with the current passing around the coil 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 current 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 way of Z, D, 3, B, and X. The current can then
be rapidly reversed by raising and lowering N.