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Various methods have been devised for overcoming this objection, the most prominent of the resulting cells being known as the Fuller, Gordon and Edison-Lalande Cells.

FIG. 7. Edison Cell.

The liquid excitant of the Gordon and Edison-Lalande cells is a strong solution of sodium hydroxide. The positive pole of these cells is a block of compressed copper oxide and the negative a pair of zinc plates. In the Gordon cell the positive is enclosed in a porous chamber.

FIG. 8. Dry Cell.

One of the best known forms of cell is the dry cell. It consists of an outer shell of zinc forming the negative electrode and a central rod of carbon as the positive. The active agent of the cell is a paste composed principally of sal ammoniac lining the interior of the zinc shell. The depolarizing agent of the cell is manganese dioxide mixed with crushed carbon and packed tightly around the carbon rod. The cell is not as its name implies perfectly dry inside, but the chemicals are in paste form. The cell is sealed at the top by a bituminous compound making the cell air tight and portable. Dry cells are only successful for intermittent work, that is, where they are not required to deliver a heavy current continuously. They deteriorate after long standing because the moisture evaporates. Dry cells, however, are a very convenient source of current where the demand is not too great and portability is desired.