An examination of Fig. 63 shows that the dry cell has all of the essential elements of the LeClanché cell. The materials of which the electrodes are made are the same and the porous cup of the disk LeClanché cell is represented in the dry cell by the blotting-paper cylinder, which separates the zinc from the carbon electrode. The positively charged electrode must not be considered as merely the carbon plate or rod alone, but rather the carbon rod with its surrounding mixture of peroxide of manganese and broken carbon. Such being the case, it is obvious that the separation between the electrodes is very small, while the surface presented by both electrodes is very large. As a result, the internal resistance of the cell is small and the current which it will give on a short circuit is correspondingly large. A good cell of the two and three-quarters by six-inch size will give eighteen or twenty amperes on short-circuit, when new.

As the action of the cell proceeds, zinc chloride and ammonia are formed, and there being insufficient water to dissolve the ammonia, there results the formation of double chlorides of zinc and ammonium. These double chlorides are less soluble than the chlorides and finally occupy the pores of the paper lining between the electrolyte and the zinc and greatly increase the internal resistance of the cell. This increase of resistance is further contributed to by the gradual drying out of the cell as its age increases.

Within the last few years dry batteries have been so perfected mechanically, chemically, and electrically that they have far greater outputs and better recuperative power than any of the other types of LeClanché batteries, while in point of convenience and economy, resulting from their small size and non-breakable, non-spillable features and low cost, they leave no room for comparison.

Closed-Circuit Cells. Gravity-Cell:—Coming now to the consideration of closed-circuit or constant-current cells, the most important is the well-known gravity, or blue-stone, cell, devised by Daniell. It is largely used in telegraphy, and often in telephony in such cases as require a constantly flowing current of small quantity. Such a cell is shown in Fig. 64.

The elements of the gravity cell are electrodes of copper and zinc. The solution in which the copper plate is immersed is primarily a solution of copper sulphate, commonly known as blue-stone, in water. The zinc plate after the cell is in action is immersed in a solution of sulphate of zinc which is formed around it.

The glass jar is usually cylindrical, the standard sizes being 5 inches diameter and 7 inches deep; and also 6 inches diameter and 8 inches deep. The copper electrode is of sheet copper of the form shown, and it is partly covered with crystals of blue-stone or copper sulphate. Frequently, in later forms of cells, the copper electrode consists merely of a straight, thick, rectangular bar of copper laid horizontally, directly on top of the blue-stone crystals. In all cases a rubber-insulated wire is attached by riveting to the copper electrode, and passes up through the electrolyte to form the positive terminal.

Fig. 64. Gravity Cell
[View full size illustration.]

The zinc is, as a rule, of crowfoot form, as shown, whence this cell derives the commonly applied name of crowfoot cell. This is essentially a two-fluid cell, for in its action zinc sulphate is formed, and this being lighter than copper sulphate rises to the top of the jar and surrounds the zinc. Gravity, therefore, serves to keep the two fluids separate.