The action of this cell is as follows:  The sulphuric acid attacks the zinc and forms zinc sulphate, liberating hydrogen.  The hydrogen attempts to pass to the carbon plate as usual, but in so doing it meets with the oxygen of the chromic acid and forms water therewith.  The remainder of the chromic acid combines with the sulphuric acid to form chromium sulphate.

[Illustration:  Fig 65.  Fuller Cell]

The mercury placed in the bottom of the porous cup with the zinc keeps the zinc in a state of perpetual amalgamation.  This it does by capillary action, as the mercury spreads over the entire surface of the zinc.  The initial amalgamation, while not absolutely essential, helps in a measure this capillary action.

In another well-known type of the Fuller battery the carbon is a hollow cylinder, surrounding the porous cup.  In this type the zinc usually took the form of a long bar having a cross-shaped section, the length of this bar being sufficient to extend the entire depth of the porous cup.  This type of cell has the advantage of a somewhat lower internal resistance than the standard form just described.

Should the electrolyte become 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  Should this occur, some of the solution should be withdrawn and dilute sulphuric acid inserted in its place and the crystals which have formed on the carbon should be carefully washed off.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.