Chapter 17

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.)

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.

APPARATUS 132.

_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 8-1/2 x 4 x 7/8 in. The spools are 1-3/4 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.

234. Connections. If you join a Wheatstone's bridge, for example, with U and V (Fig. 106), the resistance added will be but 1 Ohm; if you join with U and W, the coils A and B will be in the circuit and make 3 Ohms resistance; if V and X, 4 Ohms; if V and Y, 9 Ohms; if U and Z, the whole, or 20 Ohms.

APPARATUS 133.

_235. Resistance Coils._ For use in some experiments in comparing the resistance, diameter, lengths, etc., of wires (See text-book), it is very handy to have coils made a certain number of meters long. (The meter is a French unit of measure and represents 39.3705 of our inches). German-silver wire has a much greater resistance than copper wire of the same size and length.

(a) Make a coil (See App. 131 for method) containing 1 meter of No. 30 German-silver wire.

(b) Make a coil with 2 meters No. 30 German-silver wire.

(c) Make one with 2 meters of No. 28 German-silver wire.

(d) Make one with 20 meters of No. 30 copper wire.

The above wire must be insulated if it is to be wound upon spools. Bare wire may be arranged on boards or racks so that the current may not be short circuited.