The Hellesen dry cell is like the "E. C. C.," but contains a
hollow carbon, and is packed with sawdust in a millboard case. The
Leclanche-Barbier dry cell is a modification of the Leclanche wet
cell, having a paste of sal-ammoniac instead of a solution.

All the foregoing cells are called "primary," because they are
generators of electricity. There are, however, batteries known as
"secondary," which store the current as the Leyden jar stores up
the discharge from an electrical machine.

In the action of a primary cell, as we have seen, water is split
into its constituent gases, oxygen and hydrogen. Moreover, it was
discovered by Carlisle and Nicholson in the year 1800 that the
current of a battery could decompose water in the outer part of
the circuit. Their experiment is usually performed by the.
apparatus shown in figure 20, which is termed a voltameter, and
consists of a glass vessel V, containing water acidulated with a
little sulphuric acid to render it a better conductor, and two
glass test-tubes OH inverted over two platinum strips or
electrodes, which rise up from the bottom of the vessel and are
connected underneath it to wires from the positive and negative
poles of the battery C Z. It will be understood that the current
enters the water by the positive electrode, and leaves it by the
negative electrode.

When the power of the battery is sufficient the water in the
vessel is decomposed, and oxygen being the negative element,
collects at the positive foil or electrode, which is covered by
the tube O. The hydrogen, on the other hand, being positive,