342. Directions. (A) Measure the resistance of the coil as before, [Exp. 129]. The result should nearly agree with that of [Exp. 129], provided connections, etc., are the same.
(B) Remove the coil from the bridge, hold it about a foot above a lamp or stove, to warm it thoroughly, but do not heat it enough to injure the covering. It will take a minute or so to warm it so that the heat will get to the inside also.
(C) Replace the coil, measure its resistance, and compare the result with its resistance when cold. Does heat increase or decrease the resistance of a copper wire?
343. Effect of Heat upon Resistance. Although there was but the fraction of an ohm difference in the resistances of the hot and cold coil, it was evident that changes of temperature affect the conducting power of copper. This is true of all metals; but German silver and other alloys are much less affected than pure metals, so they are used in making standard resistance coils. The resistance of liquids that can be decomposed by the electric current decreases as the temperature rises. Carbon acts like the liquids, while the resistance of metals increases as their temperature rises.
EXPERIMENT 139. To measure the resistance of a wire by the method of "substitution."
Apparatus. The coil of No. 24 wire (No. 89), the resistance of which has been measured, but which will be considered an unknown resistance, X; G V, 2-F C, M P, connecting wires, etc., previously used; rheostat ([§ 344]). Arrange as in [Fig. 106] first, then as in [Fig. 107].
344. Simple Rheostat. The No. 28 and No. 30 G-s wires stretched upon the board ([Fig. 96]), make a convenient form of rheostat. The resistance per cm. being known from the results of [Exp. 133] and 136, the resistance for any number of cm. is easily found. The 10-cm. divisions should be divided into centimeters. These spaces may be marked off from the rule (No. 88).
