Increasing r from 2 to 3, as before, etc., we find that C = 1 divided by 203 = .00492 ampere.

The above shows clearly (A) that the value of C is changed considerably by changes in r when R is small, and (B) that changes in r produce very slight changes in C when R is large. Review your results of [Exps. 142]–[144]. (See Elementary Electrical Examples.)

363. Arrangement of Cells and Current Strength. We have seen that internal resistance affects current strength. In joining cells, then, attention must be given to the internal resistance as well as to the E. M. F. of the combination.

364. Cells in Series. It has been shown by careful experiments that the E. M. F. of two cells joined in[148] series ([Fig. 110]) is equal to the sum of the E. M. F. of each. Ten cells, joined in series, have ten times the E. M. F. of one cell, provided they have the same E. M. F. As the Zn of one is joined to the Cu of the other, the current is obliged to pass through one solution after the other; that is, the internal resistance of the two in series is equal to the sum of their internal resistances. Ten cells, joined in series, have ten times the internal resistance of one cell, provided they have equal internal resistances.

Fig. 110.

Fig. 111.

365. Cells Abreast. When the positive plates are joined together and the negative plates are also joined together ([Fig. 111]), the cells are said to be abreast, in parallel, or in multiple arc. It has been shown that two cells of equal strength, joined abreast, have the same E. M. F. as one cell. The two Cu plates, being joined, must have the same potential; all the Zn plates have the same potential, so the difference of potential at the terminals of the combination is the same as that at the terminals of a single cell.

In two cells abreast ([Fig. 111]) the current has two liquid paths, side by side, to get from Cu to Zn; this makes the internal resistance one-half that of one cell, provided their internal resistances are equal. Ten cells, of equal internal resistance, when joined abreast, have one-tenth the internal resistance of one cell.