273. Liquid Batteries. In liquid batteries, electric current is generated by means of direct chemical action between an acid and two other substances, one more easily attacked by the acid than the other (Fig. 153), such as zinc and copper. This forms a simple cell, one form of primary battery. When the chemicals and metals in a primary battery are exhausted, they can be replaced with new metal or solution.
Fig. 152. Electric generator.
274. A Dry-Cell Battery. A dry-cell is another form of battery. In these, the moisture of the acid substance is absorbed by some material like plaster-of-Paris flour or blotting paper, so that it can act on the metals or carbon in the cell and still make a cell easily transportable. The absorbed moisture in dry cells slowly evaporates, and then they become worthless. These batteries are usually thrown away after they have been used and have ceased to generate electricity.
275. Storage Batteries. Storage batteries differ from primary batteries in that current must be supplied to them from some outside source, such as a dynamo. They can be recharged again after the current in them has been used (Fig. 154).
Fig. 153. Primary battery.
Fig. 154. Storage battery.
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