lectric batteries and cells are
but a modification of that used by Volta and called a voltaic cell. A
strip of copper and a strip of zinc are placed in a glass containing
dilute sulphuric acid, a solution composed of oxygen, hydrogen,
sulphur, and water. As soon as the plates are immersed in the acid
solution, minute bubbles of gas rise from the zinc strip and it begins
to waste away slowly. The solution gradually dissolves the zinc and at
the same time gives up some of the hydrogen which it contains; but it
has little or no effect on the copper, since there is no visible
change in the copper strip.

If, now, the strips are connected by means of metal wires, the zinc
wastes away rapidly, numerous bubbles of hydrogen pass over to the
copper strip and collect on it, and a current of electricity flows
through the connecting wires. Evidently, the source of the current is
the chemical action between the zinc and the liquid.

Mere inspection of the connecting wire will not enable us to detect
that a current is flowing, but there are various ways in which the
current makes itself evident. If the ends of the wires attached to the
strips are brought in contact with each other and then separated, a
faint spark passes, and if the ends are placed on the tongue, a twinge
is felt.

282. Experiments which grew out of the Voltaic Cell. Since chemical
action on the zinc is the source of the current, it would seem
reasonable to expect a current if the cell consisted of two zinc
plates instead of one zinc plate and one copper plate. But when the
copper strip is replaced by a zinc strip so that the cell consists of
two similar plates, no current flows between them. In this case,
chemical action is expended in heat rather than in the production of
electricity and the liquid becomes hot. But if carbon and zinc are
used, a current is again produced, the zinc dissolving away as before,
and bubbles collecting on the carbon plate. By experiment it has been
found that many different metals may be employed in the construction
of an electric cell; for example, current may be obtained from a cell
made with a zinc plate and a platinum plate, or from a cell made with
a lead plate and a copper plate. Then, too, some other chemical, such
as bichromate of potassium, or ammonium chloride, may be used instead
of dilute sulphuric acid.

Almost any two different substances will, under proper conditions,
give a current, but the strength of the current is