copper, the current grows weaker, and the cell is said to
"polarise." It follows that a simple water cell is not a good
arrangement for the supply of a steady current.

The Daniell cell is one of the best, and gives a very constant
current. In this battery the copper plate is surrounded by a
solution of sulphate of copper (Cu SO4), which the hydrogen
decomposes, forming sulphuric acid (H2SO4), thus taking itself out
of the way, and leaving pure copper (Cu) to be deposited as a
fresh surface on the copper plate. A further improvement is made
in the cell by surrounding the zinc plate with a solution of
sulphate of zinc (Zn SO4), which is a good conductor. Now, when
the oxide of zinc is formed by the oxygen uniting with the zinc,
the free sulphuric acid combines with it, forming more sulphate of
zinc, and maintaining the CONDUCTIVITY of the cell. It is only
necessary to keep up the supply of zinc, water, and sulphate of
copper to procure a steady current of electricity.

The Daniell cell is constructed in various ways. In the earlier
models the two plates with their solutions were separated by a
porous jar or partition, which allowed the solutions to meet
without mixing, and the current to pass. Sawdust moistened with
the solutions is sometimes used for this porous separator, for
instance, on board ships for laying submarine cables, where the
rolling of the waves would blend the liquids.

In the "gravity" Daniell the solutions are kept apart by their
specific gravities, yet mingle by slow diffusion. Figure 15
illustrates this common type of cell, where Z is the zinc plate in
a solution of sulphate of zinc, and C is the copper plate in a