(D) Change the direction of the current through G V; to do this, change wire, 1, from M P 2 to M P 1, and wire 5 to M P 2. This must be done without disturbing G V.
(E) Repeat (A), (B), and (C), and record the deflections for the different lengths.
(F) Get the average deflections.
(G) Take, for future use, the deflection produced without G-S W being in the circuit. Swing the end of wire, 3, that is joined to Y, around to M P 2. The current will then pass simply through G V. Record deflection in col. marked O.
Note. It is best to do the next experiment at once with the same cell, so that the results of the two experiments can be compared. In case this is impossible, get your cell to produce the same deflection when you use it again, as shown in col. O, [Fig. 97]. You can regulate the deflection of the needle of G V by varying the strength and quantity of the acid in P C.
317. Discussion. The resistance of a wire evidently depends ([Exp. 121]) upon its length. The exact relation between resistance and length cannot be seen from these results, however, which are used in the next experiment. It will be shown later that in a wire, other things remaining the same, the resistance varies directly as its length.
EXPERIMENT 122. To find what effect the size (area of cross-section) of a wire has upon its electrical resistance.
Apparatus. Same as in last experiment, with one change, however. Replace the No. 30 G-silver wire with a No. 28 G-silver wire (No. 82), or, what is better still, fasten it to another set of binding-posts on the board and leave the No. 30 for future use. The two should be stretched side by side for constant use.
318. Directions. (A) See that your cell is in the same condition as for [Exp. 121]; that is, it should produce the same deflection of the needle of G V as before, when the two, only, are in the circuit. (See [Exp. 121], G.) The deflection may be changed by changing the strength and quantity of the dilute acid and copper solution.