and one-half inches of the top of the porous cup. About two
teaspoonfuls of salt are then placed in the porous cup and sufficient
soft water added to bring the level of the liquid within the porous
cup even with the level of the electrolyte in the jar surrounding the
cup. The carbon is then placed through the slot in the cover, and the
wire from the zinc is passed through the hole in the cover provided
for it, and the cover is allowed to fall in place. The cell is now
ready for immediate use.

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.