the De la Rue cell by substituting a solution of caustic potash
for the ammonium chloride, and his battery has been used for
"star" lights, that is to say, the tiny electric lamps of the
ballet. The Schanschieff battery, consisting of zinc and carbon
plates in a solution of basic sulphate of mercury, is suitable for
reading, mining, and other portable lamps.

The Latimer Clark "standard" cell is used by electricians in
testing, as a constant electromotive force. It consists of a pure
zinc plate separated from a pool of mercury by a paste of
mercurous proto-sulphate and saturated solution of sulphate of
zinc. Platinum wires connect with the zinc and mercury and form
the poles of the battery, and the mouth of the glass cell is
plugged with solid paraffin. As it is apt to polarise, the cell
must not be employed to yield a current, and otherwise much care
should be taken of it.

Dry cells are more cleanly and portable than wet, they require
little or no attention, and are well suited for household or
medical purposes. The zinc plate forms the vessel containing the
carbon plate and chemical reagents. Figure 19 represents a section
of the "E. C. C." variety, where Z is the zinc standing on an
insulating sole I, and fitted with a connecting wire or terminal T
(-), which is the negative pole. The carbon C is embedded in black
paste M, chiefly composed of manganese dioxide, and has a binding
screw or terminal T (+), which is the positive pole. The black
paste is surrounded by a white paste Z, consisting mainly of lime
and sal-ammoniac. There is a layer of silicate cotton S C above
the paste, and the mouth is sealed with black pitch P, through
which a waste-tube W T allows the gas to escape.