Arrangement of cells
A group of cells connected together is called a battery. Normally, cells are connected in series, the positive of one being connected to the negative of the next and so on (Fig. 35.7). On occasions, however, they may be connected in parallel, i.e., all the
positives connected together and the same with the negatives (Fig. 35.8).
When maximum current is required from a given number of cells the arrangement used will depend on the external circuit resistance. Generally speaking, series connection is used when the circuit resistance is high compared with that of the cells and parallel connection when it is low. This principle is illustrated in the worked example on page 414. When cells are in series the. total e.m.f of the battery is equal to the sum of the separate e.rn.f.s, and the internal resistance is equal to the sum of the separate internal resistances of the cells. When cells of equal e.m.f and internal resistance are connected in parallel the resultant e.m.f. is the same as that of one cell only and the internal resistance of the battery is calculated from the formula for resistors in parallel. One advantage of connecting cells in parallel is that there is less drain on the cells, since they share the total current, whereas with series connection the same main current is supplied by each cell. Cells should never be left connected in parallel when not in use, for if the e.m.f. of one is slightly greater than that of another current will circulate in the battery itself and the cells become exhausted. This cannot happen when they are in series.