Part 14 (2/2)

CHAPTER XVI.

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 pa.s.sing 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.

[Ill.u.s.tration: Fig. 103.]

APPARATUS 129.

_225. Current Reverser._ Fig. 103. The base is 5 4 7/8 in. To this are fastened four metal straps, A, B, C, and D. These may be made of bra.s.s, 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 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 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 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 pa.s.sed 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 pa.s.s 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 pa.s.s 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 pa.s.s out of Y or Z to the galvanometer at will by pressing down A or B.

APPARATUS 130.

[Ill.u.s.tration: Fig. 104.]

_228. Current Reverser._ Fig. 104. The wooden base is 7 5 7/8 in. To this are fastened two bra.s.s or tin straps, C and D, 5 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 3 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, pa.s.s 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 pa.s.s along A to 4, before it can find a chance to escape. It will pa.s.s 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 pa.s.s 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.

CHAPTER XVII.

RESISTANCE COILS.

APPARATUS 131.

_230. Resistance Coils._ Fig. 105. For experiments in resistance (See text-book), a set of standard resistances is necessary. There are many ways in which the resistances may be made; you can arrange them upon a long board, upon a rack, or wind the wires around spools. We generally speak of resistance coils. The Ohm is taken as the standard. If you use copper wire, you may take 9 ft. 9 in. of No. 30 insulated wire as your standard Ohm. You could, of course, take any other length of any size as your standard, but it will be best to make your coils with a certain number of Ohms resistance. If you have no No. 30 wire, you may use 39 ft. 1 in. of No. 24 insulated copper wire for 1 Ohm. (See wire tables in text-book.)

[Ill.u.s.tration: Fig. 105.]

231. To avoid the magnetic effect (See resistance coils, in text-book), the wire should be measured off, then doubled, before winding it upon the spools. The wire may be held to the spool with paraffine. Fig. 105 shows how the doubled wire looks on the spool, a few turns only being shown. Do not use any nails or other iron in connection with the coils proper.

232. By making 4 coils having, respectively, 1, 2, 2, and 5 Ohms resistance, you will be able to use any number of Ohms from 1 to 10.

These will be very handy in connection with a ”Wheatstone's bridge” for comparing resistances. (See text-book for experiments). The coils should be mounted upon a base with proper binding-posts, so that one or more coils can be used at a time. (See App. 132.) For the 2-Ohm coil use, of course, twice as much of the same kind of wire as for the 1-Ohm coil.

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