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.
_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.)
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.