_=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). [Illustration: Fig. 106.] =345. Directions.= (A) Be sure that 2-F C gives a constant current, shown by the uniform deflection at G V, when arranged as in Fig. 106. Do not use a cell that quickly polarizes. The coil, X, forms a part of the circuit; it is joined to wires, 1 and 2, by means of metal plates, so that it may be quickly removed without disturbing either G V or 2-F C. Carefully read the deflection at G V. (B) Remove X from the circuit, and join the free end of wire, 2, to binding-post, X, and the free end of wire, 1, to a small piece of sheet copper, which can be firmly pressed upon the G-s wire to make a contact. Move this along on the G-s wire until the deflection produced equals that of part (A), remembering that the longer the G-s wire in the circuit the less the deflection. Make two or three trials, as one or two cm. difference in length make but a little difference in the deflection. Note the number of cm. of G-s wire used, the resistance of which must equal that of the coil, X. (C) Find the resistance of X by multiplying the length just found by the resistance of each cm., and compare the result with the value found by using the bridge method directly. [Illustration: Fig. 107.] =EXPERIMENT 140. To measure the E. M. F. of a cell by comparison with the two-fluid cell.= _Apparatus._ Rheostat (§ 344); the two-fluid cell, 2-F C (Exp. 113), the E. M. F. of which may be taken as 1 volt; dry cell, D C; galvanoscope, G V. Arrange first as in Fig. 107.