1. The amount of chemical action in a cell is proportional to the quantity of electricity that passes through it.
One coulomb of electricity in passing through the cell liberates .000010352 of a gramme of hydrogen, and causes .00063344 of a gramme of zinc to dissolve in the acid.
2. The amount of chemical action is equal in each cell of a battery connected in series.
Requirements of a Good Cell.—The several conditions which should be fulfilled by a good cell are as follows:
1. Its electromotive force should be high and constant; 2. Its internal resistance should be small; 3. It should be perfectly quiescent when the circuit is open; 4. It should give a constant current, and therefore must be free from polarization, and not liable to rapid exhaustion; 5. It should be easily cared for, and if possible, should not emit corrosive fumes; 6. It should be cheap and of durable materials.
Single and Two Fluid Cells.—The distinction between a single and a two fluid cell has already been given. The single fluid cell of Volta with its zinc and copper plates represents the simplest form of primary cell.
In the two fluid cell, the positive (zinc) plate is immersed in the exciting liquid (usually dilute sulphuric acid) and is decomposed by the action upon it, while the negative plate is placed in the liquid depolarizer which is decomposed by the hydrogen arrested by it, thus preventing polarization.
In some forms of cell, the two liquids are separated by a porous partition of unglazed earthenware, which, while it prevents the liquids mixing except very slowly, does not prevent the passage of hydrogen and electricity.
Complete depolarization is usually obtained also in single fluid cells, having in addition a depolarizing solid body, such as oxide of manganese, oxide of copper, or peroxide of lead, in contact with the carbon pole. Such cells really do not belong to the single fluid cells, and are considered in the two fluid class.