ecome supersaturated by virtue of the battery
being neglected or too heavily overworked, a set of secondary
reactions will occur in the cell, resulting in the formation of the
yellow crystals upon the carbon. This seriously affects the e.m.f. of
the cell and also its internal resistance. Should this occur, some of
the solution should be withdrawn and dilute sulphuric acid inserted in
its place and the crystals which have formed on the carbon should be
carefully washed off. Should the solution lose its orange tint and
turn blue, it indicates that more bichromate of potash or bichromate
of sodium is needed. This cell gives an electromotive force of 2.1
volts and a very large current when it is in good condition, since its
internal resistance is low.

The Fuller cell was once largely used for supplying current to
telephone transmitters at subscribers' stations, where very heavy
service was demanded, but the advent of the so-called common-battery
systems, in some cases, and of the high-resistance transmitter, in
other cases, has caused a great lessening in its use. This is
fortunate as the cell is a "dirty" one to handle and is expensive to
maintain.

The Fuller cell still warrants attention, however, as an available
source of current, which may be found useful in certain cases of
emergency work, and in supplying special but temporary needs for
heavier current than the LeClanche or gravity cell can furnish.

Lalande Cell:--A type of cell, specially adapted to constant-current
work, and sometimes used as a central source of current in very small
common-battery exchanges is the so-called _copper oxide_, or _Lalande
cell_, of which the Edison and the Gordon are types. In all of these
the negatively charged element is of zinc, the positively charged
element a mass of copper oxide, and the electrolyte a solution of
caustic potash in water. In the Edison cell the copper oxide is in the
form of a compressed slab which with its connecting copper support
forms the electrode. In the Gordon and other cells of this type the
copper oxide is contained loosely in a perforated cylinder of sheet
copper. The copper oxide serves not only as an electrode, but also as
a depolarizing agent, the liberated hydrogen in the electrolyte
uniting with the oxygen of the copper oxide to form water, and leaving
free metallic copper.

On open circuit the elements are not attacked, therefore there is no
waste of material while the cell is not in use. T