plate is the _positive_ plate, because it sends forth _positive_ ions
in the electrolyte, and, therefore, itself becomes _negatively_
charged, one will have the proper basis always to determine the
direction of the current flow, which is the important thing.

_Polarization._ If the simple cell already described have its
terminals connected by a wire for some time, it will be found that the
current rapidly weakens until it ceases to be manifest. This weakening
results from two causes: first, the hydrogen gas which is liberated in
the action of the cell is deposited in a layer on the copper plate,
thereby covering the plate and reducing the area of contact with the
liquid. This increases the internal resistance of the cell, since
hydrogen is a non-conductor. Second, the plate so covered becomes in
effect a hydrogen electrode, and hydrogen stands high as an
electro-positive element. There is, therefore, actual reduction in the
electromotive force of the cell, as well as an increase in internal
resistance. This phenomenon is known as polarization, and in
commercial cells means must be taken to prevent such action as far as
possible.

The means by which polarization of cells is prevented or reduced in
practice may be divided into three general classes:

First--_mechanical means_. If the hydrogen bubbles be simply
brushed away from the surface of the electrode the resistance and
the counter polarity which they cause will be diminished. The
same result may be secured if air be blown into the solution
through a tube, or if the liquid be kept agitated. If the surface
of the electrode be roughened or covered with points, the bubbles
collect more freely at the points and are more quickly carried