Dry cells (Fig. 75) are not actually dry. They contain the same ingredients as the Leclanche cell, but instead of containing a fluid electrolyte they have the solution absorbed in a plastic mass of manganese dioxide and plaster of Paris or other inactive substances.

The great advantage of the dry cell lies in the fact that the liquid will not spill out under any conditions and there are no vapors arising from the cell, as it is almost invariably sealed. Dry cells have the disadvantage, however, of having a very high internal resistance, because the electrolyte cannot so readily carry the current when in this form as it can when fluid. Furthermore, the small amount of liquid present and the method of construction do not allow the free escape of the gases which form when the cell is in operation. For this reason, the cell becomes polarized very soon and is satisfactory only where intermittent service is needed. It should not be used where the current must flow continuously for any length of time.

Fig. 75   Dry Cell.

Fig. 75 - Dry Cell.

In order that the internal resistance of the cells may be reduced to its lowest point, the zinc and carbon are arranged to present as great a surface as possible and to be as near together as circumstances will allow. This arrangement affords a large conductor of short length for the current to flow through inside the cell. The carbon should be as porous as possible, as it can then absorb a great amount of oxygen and thus neutralize the hydrogen gas produced by the cell when in operation and prevent the cell from polarizing as soon as it would if there were no oxygen present to combine with the hydrogen.