tion.

Secondary cells, commonly called _storage cells_ and _accumulators_,
consist always of two inert plates of metal, or metallic oxide,
immersed in an electrolyte which is incapable of acting on either of
them until a current has first been passed through the electrolyte
from one plate to the other. On the passage of a current in this way,
the decomposition of the electrolyte is effected and the composition
of the plates is so changed that one of them becomes electro-positive
and the other electro-negative. The cell is then, when the _charging_
current ceases, capable of acting as a voltaic cell.

This chapter is devoted to the primary cell or battery alone.

Types of Primary Cells. Primary cells may be divided into two
general classes: first, those adapted to furnish constant current; and
second, those adapted to furnish only intermittent currents. The
difference between cells in this respect rests largely in the means
employed for preventing or lessening polarization. Obviously in a cell
in which polarization is entirely prevented the current may be allowed
to flow constantly until the cell is completely exhausted; that is,
until the zinc is all eaten up or until the hydrogen is exhausted from
the electrolyte or both. On the other hand some cells are so
constituted that polarization takes place faster than the means
intended to prevent it can act. In other words, the polarization
gradually gains on the preventive means and so gradually reduces the
current by increasing the resistance of the cell and lowering its
electromotive force. In cells of this kind, however, the arrangement
is such that if the cell is allowed to rest, that is, if the external
circuit is opened, the depolarizing agency will gradually act to
remove the hydrogen from the unattacked electrode and thus place the
cell in good condition for use again.

Of these two types of primary cells the intermittent-current cell is
of far greater use in telephony than the constant-current cell. This
is because the use of primary batteries in telephony is, in the great
majority of cases, intermittent, and for that reason a cell which will
give a strong current for a few minutes and which after such use will
regain practically all of its initial strength and be ready for use
again, is more desirable than one which will give a weaker current
continuously throughout a long period of time.

Since the cells which are adapted to give constant current are
common