arge
with respect to the copper. When the two plates are connected by a
wire, a current flows from the copper to the zinc plate through the
metallic path of the wire, just as is to be expected when any
conductor of relatively high electrical potential is joined to one of
relatively low electrical potential. Ordinarily, when one charged body
is connected to another of different potential, the resulting current
is of but momentary duration, due to the redistribution of the charges
and consequent equalization of potential. In the case of the simple
cell, however, the current is continuous, showing that some action is
maintaining the charges on the two plates and therefore maintaining
the difference of potential between them. The energy of this current
is derived from the chemical action of the acid on the zinc. The cell
is in reality a sort of a zinc-burning furnace.

In the action of the cell, when the two plates are joined by a wire,
it may be noticed that the zinc plate is consumed and that bubbles of
hydrogen gas are formed on the surface of the copper plate.

_Theory_. Just why or how chemical action in a voltaic cell results
in the production of a negative charge on the consumed plate is not
known. Modern theory has it that when an acid is diluted in water the
molecules of the acid are split up or _dissociated_ into two
oppositely charged atoms, or groups of atoms, one bearing a positive
charge and the other a negative charge of electricity. Such charged
atoms or groups of atoms are called _ions_. This separation of the
molecules of a chemical compound into positively and negatively
charged ions is called _dissociation_.

Thus, in the simple cell under consideration the sulphuric acid, by
dissociation, splits up into hydrogen ions bearing positive charges,
and SO_{4} ions bearing negative charges. The solution as a whole is
neutral in potential, having an equal number of equal and opposite
charges.

[Illustration: Fig. 60. Simple Voltaic Cell]

It is known that when a metal is being dissolved by an acid, each atom
of the metal which is torn off by the solution leaves the metal as a
positively charged ion. The carrying away of positive charges from a
hitherto neutral body leaves that body with a negative charge. Hence
the zinc, or _consumed_ plate, becomes negatively charged.

In the chemical attack of the sulphuric acid on the zinc, the positive
hydrogen ions are liberated, due to the affinity of the negative