shown in Fig. 64.

The elements of the gravity cell are electrodes of copper and zinc.
The solution in which the copper plate is immersed is primarily a
solution of copper sulphate, commonly known as blue-stone, in water.
The zinc plate after the cell is in action is immersed in a solution
of sulphate of zinc which is formed around it.

The glass jar is usually cylindrical, the standard sizes being 5
inches diameter and 7 inches deep; and also 6 inches diameter and 8
inches deep. The copper electrode is of sheet copper of the form
shown, and it is partly covered with crystals 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]