=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. (B) Find the average deflection of the needle with the 2 meters of No. 28 G-s wire in the circuit, arranged as in Fig. 96. (C) Compare this average deflection with the results obtained in Exp. 121, in order to find what length of the No. 30 wire has the same resistance as 2 meters of No. 28 wire. To find how many times greater one length is than another, we divide the larger length by the smaller; hence, to find the relation between the two lengths of wire that gave the same deflection,--lengths of equal resistance,--we divide the 200 centimeters (the length of the No. 28) by the length of No. 30 found as directed. (D) From the wire tables it will be found that the area of cross-section of No. 28 wire is about 1.59 times that of No. 30 wire. How does this quotient, or ratio, compare with that found in part (C)? What is the relation between the area of cross-section of a wire and its resistance? (See § 319, also Exp. 136.) _=319. Discussion.=_ If we find that a certain wire, X, which is 576 feet long, has the same resistance as a shorter one, Y, 360 feet long, we see (576 divided by 360) that the ratio of their lengths is 1.6. This means that the longer one, X, is 1.6 times as good a _conductor_ as Y; or, in other words, that the _resistance_ of Y is 1.6 times that of X. It is easier for water to flow through a large pipe than it is through a small one. The same general principle is true of electricity. A large wire offers less resistance to the current than a small one of the same material. If one wire is twice the size of another of equal length, it will be twice as good a conductor as the other; that is, it will have one-half the resistance of the smaller, provided they are of the same material. (See Laws.) =EXPERIMENT 123. To compare the resistance of a divided circuit with the resistance of one of its branches.= _Apparatus._ Same as in last experiment. Arrange as in Fig. 98.