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.
Fig. 106.
233. Resistance Coils. Fig. 106. The construction of one coil is given in [App. 131]. To have the set of coils so that they can be easily used, place the spools upon a base which, in the model, is 8½ × 4 × ⅞ in. The spools are 1¾ in. apart, center to center, and should be glued to the base. Fig. 106 is a plan of the apparatus. U, V, etc., are binding-posts like [App. 46]. The figures between them show how many Ohms resistance there are in the coil above. The coils A, B, C, D, and E are wound respectively for 1, 2, 2, 5 and 10 Ohms.