Fig. 103.
225. Current Reverser. Fig. 103. The base is 5 × 4 × ⅞ 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 ⅛ in. holes punched in it, their positions being shown by the screw-heads and screw-eye binding-posts.
Construction. C is 3¾ × ½ 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 × ¾ 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 ⅛ in. holes in the end of A are about ¾ 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 3¾ × ½ in. It is fastened at one end by a screw, and at the other end by X. D is bent about ¾ in. from each end, so that its middle part stands above the base about ¼ 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.