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.)

[Illustration: 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.

APPARATUS 132.

[Illustration: 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 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 resi