solution of sulphate of copper, fed by crystals of the "blue
vitriol." The wires to connect the plates are shown at WW. It
should be noticed that the zinc is cast like a wheel to expose a
larger surface to oxidation, and to reduce the resistance of the
cell, thus increasing the yield of current. The extent of surface
is not so important in the case of the copper plate, which is not
acted on, and in this case is merely a spiral of wire, helping to
keep the solutions apart and the crystals down. The Daniell cell
is much employed in telegraphy. The Bunsen cell consists of a zinc
plate in sulphuric acid, and at carbon plate in nitric acid, with
a porous separator between the liquids. During the action of the
cell, hydrogen, which is liberated at the carbon plate, is removed
by combining with the nitric acid. The Grove cell is a
modification of the Bunsen, with platinum instead of carbon. The
Smee cell is a zinc plate side by side with a "platinised" silver
plate in dilute sulphuric acid. The silver is coated with rough
platinum to increase the surface and help to dislodge the hydrogen
as bubbles and keep it from polarising the cell. The Bunsen,
Grove, and Smee batteries are, however, more used in the
laboratory than elsewhere.

The Leclanche is a fairly constant cell, which requires little
attention. It "polarises" in action but soon regains its normal
strength when allowed to rest, and hence it is useful for working
electric bells and telephones. As shown in figure 16, it consists
of a zinc rod with its connecting wire Z, and a carbon plate C
with its binding screw, between two cakes M M of a mixture of
black oxide of manganese, sulphur, and carbon, plunged in a
solution of sal-ammoniac. The oxide of manganese relieves the
carbon plate of its hydrogen. The strength of the solution is