FIG. 199.—A dry cell.
286. Dry Cells. The gravity cell, while cheap and effective, is inconvenient for general use, owing to the fact that it cannot be easily transported, and the dry cell has largely supplanted all others, because of the ease with which it can be taken from place to place. This cell consists of a zinc cup, within which is a carbon rod; the space between the cup and rod is packed with a moist paste containing certain chemicals. The moist paste takes the place of the liquids used in other cells.
FIG. 200.—A battery of three cells.
287. A Battery of Cells. The electromotive force of one cell may not give a current strong enough to ring a door bell or to operate a telephone. But by using a number of cells, called a battery, the current may be increased to almost any desired strength. If three cells are arranged as in Figure 200, so that the copper of one cell is connected with the zinc of another cell, the electromotive force of the battery will be three times as great as the E.M.F. of a single cell. If four cells are arranged in the same way, the E.M.F. of the battery is four times as great as the E.M.F. of a single cell; when five cells are combined, the resulting E.M.F. is five times as great.