A small local action takes place in the dry cell, caused by the dissimilar metals necessarily employed in soldering up the zinc cup and in soldering the terminal rod of zinc to the zinc cup proper.  This action, however, is slight in the better grades of cells.  As a result of this, and also of the gradual drying out of the moisture within the cell, these cells gradually deteriorate even when not in use—­this is commonly called shelf-wear.  Shelf-wear is much more serious in the very small sizes of dry cells than in the larger ones.

Dry cells are made in a large number of shapes and sizes.  The most useful form, however, is the ordinary cylindrical type.  These are made in sizes varying from one and one-half inches high and three-quarters inch in diameter to eight inches high and three and three-quarters inches in diameter.  The most used and standard size of dry cell is of cylindrical form six inches high and two and three-quarters inches in diameter.  The dry cell when new and in good condition has an open-circuit voltage of from 1.5 to 1.6 volts.  Perhaps 1.55 represents the usual average.

A cell of the two and three-quarters by six-inch size will give throughout its useful life probably thirty ampere hours as a maximum, but this varies greatly with the condition of use and the make of cell.  Its effective voltage during its useful life averages about one volt, and if during this life it gives a total discharge of thirty ampere hours, the fair energy rating of the cell will be thirty watt-hours.  This may not be taken as an accurate figure, however, as the watt-hour capacity of a cell depends very largely, not only on the make of the cell, but on the rate of its discharge.

An examination of Fig. 63 shows that the dry cell has all of the essential elements of the LeClanche cell.  The materials of which the electrodes are made are the same and the porous cup of the disk LeClanche 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.