=320. Directions.= (A) Note the deflection of the needle when the current passes through 1 meter of G-s wire, as shown. This will be considered as one branch of the divided circuit. (B) Still allowing the current to pass as in part (A), press a piece of copper firmly across the binding-posts X and Y, to electrically connect them, and note the reading of the needle. In this case the current divides at Z through the two branches. What is learned from the results of (A) and (B)? (C) See if you can show the same results with apparatus arranged as in Fig. 99. [Illustration: Fig. 98.] [Illustration: Fig. 99.] _=321. Discussion.=_ Two wires placed side by side as in (B), Exp. 123, really form a conductor having twice the size (area of cross section) of one of the branches. The more paths a current has in going from one place to another, the less the resistance. (See Exp. 135.) The wires are said to be in "parallel" or in "multiple arc." [Illustration: Fig. 100.] [Illustration: Fig. 101.] =EXPERIMENT 124. To study the effect of decreasing the resistance in one branch of a divided circuit.= _Apparatus._ Galvanoscope, G V (No. 58); resistance coil, R C (No. 79); two-fluid cell, 2-F C (§ 281), or a dry cell; 6 connecting wires; metal plates, M P. _Arrange_ as in Fig. 100, so that the current divides into two branches at M P 1. The branches unite at M P 2. =322. Directions.= (A) Take the reading of G V with 2 ohms resistance in the lower branch; that is, with the whole of R C in circuit. (B) Take the reading of G V with one ohm in circuit; that is, with the end of wire, 5, connected to M instead of to R. (C) Cut out R C from the lower branch by replacing it with a metal plate, thus joining wires 3 and 5. Compare the results from (A), (B), and (C), and explain. _=323. Current in Divided Circuits.=_ Let us consider a circuit like that shown in Fig. 101. If the points, C and Z, were at the same potential, no current would pass from C to Z. As the current does pass, Z must be at a lower potential than C; there is a _fall of potential_ from C to Z. If the branch, A B, has the same resistance as R X, the same amount of current will pass through each. Exp. 124 has shown that when the branches have unequal resistances, most of the current passes through the one of small resistance. If R X has a greater resistance than A B, most of the current will pass through A B. CHAPTER XIX. MEASUREMENT OF RESISTANCE. [Illustration: Fig. 102.] =EXPERIMENT 125. To study the construction and use of a simple "Wheatstone's Bridge."= _Apparatus._ Fig. 102. A Wheatstone's bridge, W B (No. 80), (§ 324); astatic galvanoscope, A G (No. 59); dry cell, D C (No. 51); key, K (No. 55); 7 wires with spring connectors, two of which, R and X, are equal in length; metal plate, M P, for connecting wires.