Apparatus. Galvanoscope, G V; cell; 2-F C; connecting wires; saucer or tumbler, S; a little sulphuric acid.
Arrange as in [Fig. 95].
313. Directions. (A) Put a little pure water in S, and see if enough current can pass through it to deflect the needle of G V. The ends of the wires, 1 and 2, should be gradually moved toward each other, the needle being watched.
(B) Put 4 or 5 drops of concentrated acid into the water; stir it, then repeat the test. What effect has the acid?
314. Internal Resistance. As found in [Exp. 120], pure water is not a good conductor of galvanic electricity. The acid in the simple cell, and in other single-fluid cells, acts upon the zinc and at the same time makes it possible for the current to pass, as it reduces the internal resistance.
As seen later, this resistance in cells is greatly diminished by bringing the plates near each other, and by increasing the surface of the plates that are in contact with the acid. The larger the plates the less the internal resistance, other things remaining the same. The internal resistance of a battery can be changed by connecting the cells differently. (See Chap. on [Arrangement of Cells].)
EXPERIMENT 121. To find what effect the length of a wire has upon its electrical resistance.