CHAPTER XVI.
THE ELECTRIC CIRCUIT.
EXPERIMENT 114. To see what is meant by "divided circuits" or "shunts."
Apparatus. The galvanoscope, G V (No. 58); astatic galvanoscope, A G (No. 59); two-fluid cell, 2-F C (see § 281); 6 wires with connectors; small thin pieces of tin or other metal, M P, for rapidly making connections (§ 226). Arrange as in Fig. 88. The wires, 1 and 4, from 2-F C, lead to the metal plates M P-A and M P-B, for convenience. The wires, 2 and 3, from G V, are also connected with these plates. The wires, 5 and 6 (dotted lines), lead from A G, to be used as directed in part (B) of the experiment. See that G V is properly placed. See that A G is adjusted.
292. Directions. (A) Without A G in place, take the reading of G V. The current now passes from Cu through 1, M P-B, 2, G V, 3, M P-A, 4 to Zn.
(B) Connect wires 5 and 6 to the plates, as shown by the dotted lines. Again take reading of G V, and compare it with the first reading. Does some of the current pass through A G?
293. Divided Circuits; Shunts. The current divides at M P-B into two parts; one part may be called a shunt of the other. The circuit is said to be divided; it has two branches. If the two ends of a wire be fastened to another as in Fig. 101, the circuit is also divided. When two or more conductors lead side by side from one point to another, they are called parallel circuits; that is, the conductors are joined in parallel.
As strong currents would injure delicate galvanometers,[116] a small part only of the current may be allowed to pass through the galvanometer by using a shunt. Fig. 89 shows such an arrangement, in which most of the current passes through the shunt, S. There are many practical uses of shunts.
EXPERIMENT 115. To see what is meant by "short circuits."
Apparatus. About the same as in Exp. 114, Fig. 88. The astatic galvanoscope is not needed; in place of it provide a short piece of metal, such as a battery-plate, or even a jack-knife. Arrange as in Fig. 88, but without A G.
294. Directions. (A) With the current passing as described in Exp. 114 (A), take the reading of G V.
(B) Lay the ends of the metal, or other thick conductor, upon M P-A and M P-B. Compare the new reading of G V with that in part (A).
(C) Remove the conductor used to short circuit G V, take the reading in degrees, then touch M P-A to M P-B; watch G V.
295. Short Circuits are very apt to occur unless care is taken. Do not allow uninsulated wires to touch each other. As shown by the above experiment, practically the whole of the current may be side-tracked by a shunt of low resistance. A galvanic cell is short-circuited by connecting the plates directly by a wire or other conductor.