=338. Directions.= (A) Using the No. 28 G-s wire on the board, as arranged for Exp. 122, measure the resistance of the 2 meters in series, the connections being made with the bridge from X and Y, Fig. 98. (B) Compare the result with that of Exp. 133. What is the relation between the length of a wire and its resistance? See Summary of Laws. (See Exp. 129, Note.) =EXPERIMENT 135.= =339. Directions.= (A) Measure the resistance of the above two meters of No. 28 G-s wire when joined in parallel. (§ 293.) The binding-posts, X and Y, can be joined by a short wire with connectors on its ends, or by clamping a thin strip across by means of spring connectors. Use the 2-ohm coil as the standard, and make proper allowances. (§ 330.) (B) From the results of Exps. 132 and 135 what can be said about the resistances of parallel circuits as compared with the resistances of the separate branches? =EXPERIMENT 136.= =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.) =EXPERIMENT 137.= =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. =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?