als of blue-stone or copper
sulphate. Frequently, in later forms of cells, the copper electrode
consists merely of a straight, thick, rectangular bar of copper laid
horizontally, directly on top of the blue-stone crystals. In all cases
a rubber-insulated wire is attached by riveting to the copper
electrode, and passes up through the electrolyte to form the positive
terminal.

[Illustration: Fig. 64. Gravity Cell]

The zinc is, as a rule, of crowfoot form, as shown, whence this cell
derives the commonly applied name of _crowfoot cell_. This is
essentially a two-fluid cell, for in its action zinc sulphate is
formed, and this being lighter than copper sulphate rises to the top
of the jar and surrounds the zinc. Gravity, therefore, serves to keep
the two fluids separate.

[Illustration: INTERIOR OF WAREHOUSE FOR TELEPHONE CONSTRUCTION
MATERIAL]

In the action of the cell, when the external circuit is closed,
sulphuric acid is formed which attacks the zinc to form sulphate of
zinc and to liberate hydrogen, which follows its tendency to attach
itself to the copper plate. But in so doing the hydrogen necessarily
passes through the solution of sulphate of copper surrounding the
copper plate. The hydrogen immediately combines with the SO_{4}
radical, forming therewith sulphuric acid, and liberating metallic
copper. This sulphuric acid, being lighter than the copper sulphate,
rises to the surface of the zinc and attacks the zinc, thus forming
more sulphate of zinc. The metallic copper so formed is deposited on
the copper plate, thereby keeping the surface bright and clean. Since
hydrogen is thus diverted from the copper plate, polarization does not
ensue.

The zinc sulphate being colorless, while the copper sulphate is of a
dark blue color, the separating line of the two liquids is easily
distinguishable. This line is called the _blue line_ and care should
be taken that it does not reach the zinc and cause a deposit of copper
to be placed thereon.

As has been stated, these two liquids do not mix readily, but they
will eventually mingle unless the action of the cell is sufficient to
use up the copper sulphate as speedily as it is dissolved. Thus it
will be seen that while the cell is free from polarization and local
action, there is, nevertheless, a deteriorating effect if the cell is
allowed to remain long on open circuit. Therefore, it should be used
when a constant current is required.

Prevention of Creeping:--Much