340. Directions. (A) Arrange the 2 meters of No. 30 G-s wire on the table or board, again ([Exp. 121], [Fig. 96]).
(B) Measure the resistance of one meter. Find the value of X approximately, and use a resistance for R that will suit. (See [Exp. 129], Note.)
(C) Divide the result by 100 to get the resistance of 1 cm. of the wire.
(D) Compare the resistance of one meter of No. 28 G-s wire, found in [Exp. 133], with the resistance of 1 meter of No. 30 G-s wire. What is the relation, then, between the size (area of cross-section) of a wire and its resistance? (See the results of [Exp. 122], and [§ 319], also Summary of Laws.)
341. Directions. (A) Measure the resistance of 2 meters of No. 30 copper wire, arranged on a board as in [Fig. 96]. (See [Exp. 129], Note.) Get the resistance of 1 meter.
(B) Compare the conductivities of copper and German silver by studying the results of Exps. 136 and 137. Which has the greater resistance? To find out how many times greater one resistance is than the other, divide the larger by the smaller.
EXPERIMENT 138. To study the effect of heat upon the resistance of metals.
Apparatus. Same as for [Exp. 126]; the coil of No. 24 wire (No. 89); a lamp or other source of heat. Arrange as in [Fig. 105].