_=364. Cells in Series.=_ It has been shown by careful experiments that the E. M. F. of two cells joined in 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. [Illustration: Fig. 110.] [Illustration: 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. =EXPERIMENT 145. To find the best way to join two similar cells when the external resistance is small.= _Apparatus._ Two simple cells using dilute sulphuric acid, with copper and zinc elements, as in Exp. 112; galvanoscope, G V; connecting wires, etc. Have the zincs well amalgamated. Remove them from the acid as soon as readings are taken. =366. Directions.= (A) Partly fill the tumblers with the acid. Join the cells in series (Fig. 110), then connect wire 1 (Fig. 110) with the left-hand binding-post of G V, and wire 2 with the middle one, thus putting the 5-turn coil into the circuit. Take the reading of G V. (B) Join the cells in multiple arc (Fig. 111), connecting them as in (A) with G V. Write down the reading, and compare it with that found in (A). (C) Take the reading with but 1 cell joined to G V. =EXPERIMENT 146. To find the best way to join two similar cells when the external resistance is large.= _Apparatus._ Same as for Exp. 145, also the rheostat containing 2 metres of No. 28 or 30 G-s wire. Arrange the G-s wire in series with the 15-turn coil of G V, as shown in Fig. 109, two simple cells being used, however, instead of 2-F C as shown. =367. Directions.= (A) Take the reading of G V when the two cells are in series (Exp. 145), the external resistance being the 15-turn coil and G-s wire.