maintained by spare crystals of sal-ammoniac lying on the bottom
of the cell, which is closed to prevent evaporation, but has a
venthole for the escape of gas.

The Bichromate of Potash cell polarises more than the Leclanche,
but yields a more powerful current for a short time. It consists,
as shown in figure 17, of a zinc plate Z between two carbon plates
C C immersed in a solution of bichromate of potash, sulphuric acid
(vitriol), and water. The zinc is always lifted out of the
solution when the cell is not in use. The gas which collects in
the carbons, and weakens the cell, can be set free by raising the
plates out of the liquid when the cell is not wanted. Stirring the
solution has a similar effect, and sometimes the constancy of the
cell is maintained by a circulation of the liquid. In Fuller's
bichromate cell the zinc is amalgamated with mercury, which is
kept in a pool beside it by means of a porous pot.

De la Rue's chloride of silver cell (fig. 18) is, from its
constancy and small size, well adapted for medical and testing
purposes. The "plates" are a little rod or pencil of zinc Z, and a
strip or wire of silver S, coated with chloride of silver and
sheathed in parchment paper. They are plunged in a solution of
ammonium chloride A, contained in a glass phial or beaker, which
is closed to suppress evaporation. A tray form of the cell is also
made by laying a sheet of silver foil on the bottom of the shallow
jar, and strewing it with dry chloride of silver, on which is laid
a jelly to support the zinc plate. The jelly is prepared by mixing
a solution of chloride of ammonium with "agar-agar," or Ceylon
moss. This type permits the use of larger plates, and adapts the
battery for lighting small electric lamps. Skrivanoff has modified