oo, is often added to effect amalgamation of the zinc.

As in the ordinary wet type of LeClanche cell, the purpose of the
manganese is to act as a depolarizer; the carbon or graphite being
added to give conductivity to the manganese and to form a large
electrode surface. It is important that the sal ammoniac, which is the
active agent of the cell, should be free from lumps in order to mix
properly with the manganese and carbon.

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 electrod