zinc element being in the form of a cylindrical cup and
forming the retaining vessel of the cell, while the carbon element is
in the form of a rod or plate and occupies a central position with
regard to the zinc, being held out of contact with the zinc, however,
at all points.

A cross-section of an excellent form of dry cell is shown in Fig. 63.
The outer casing is of zinc, formed in the shape of a cylindrical cup,
and serves not only as the retaining vessel, but as the negatively
charged electrode. The outer surface of the zinc is completely covered
on its sides and bottom with heavy pasteboard so as to insulate it
from bodies with which it may come in contact, and particularly from
the zinc cups of other cells used in the same battery. The positively
charged electrode is a carbon rod corrugated longitudinally, as shown,
in order to obtain greater surface. This rod is held in the center of
the zinc cup out of contact therewith, and the intervening space is
filled with a mixture of peroxide of manganese, powdered carbon, and
sal ammoniac. Several thicknesses of blotting paper constitute a
lining for the inner portion of the zinc electrode and serve to
prevent the manganese mixture from coming directly into contact
therewith. The cell is sealed with pitch, which is placed on a layer
of sand and sawdust mixed in about equal parts.

[Illustration: Fig. 63. Dry Cell]

The electrolyte in such cells varies largely as to quantities and
proportions of the materials employed in various types of cells, and
also varies in the method in which the elements are introduced into
the container.

The following list and approximate proportions of material will serve
as a fair example of the filling mixture in well-known types of cells.

Manganese dioxide 45 per cent
Carbon or graphite, or both 45 per cent
Sal ammoniac 7 per cent
Zinc chloride 3 per cent

Water is added to the above and a sufficient amount of mixture is
taken for each cell to fill the zinc cup about seven-eighths full when
the carbon is in place. The most suitable quantity of water depends
upon the original dryness and fineness of material and upon the
quality of the paper lining.

In some forms of dry batteries, starch or other paste is added to
improve the contact of the electrolyte with the zinc and promote a
more even distribution of action throughout the electrolyte. Mercury,
t