s a whole, and
without making extraordinary provisions to prevent the interruption of
service. It is also customary to provide charging and supply leads from
the storage battery of carrying capacity sufficient for the ultimate
requirements of the office.

=Storage Battery.= The storage battery is the power plant element which
has made common-battery systems possible. The common-battery system is
the element which has made the present wide development of telephony
possible.

A storage-battery cell is an electro-chemical device in which a chemical
state is changed by the passage of current through the cell, this state
tending to revert when a current is allowed to flow in the opposite
direction. A storage cell consists of two conductors in a solution, the
nature and the relation of these three elements being such that when a
direct current is made to pass from one conductor to the other through
the solution, the compelled chemical change is proportional to the
product of the current and its duration. When the two conductors are
joined by a path over which current may flow, a current does flow in the
opposite direction to that which charged the cell.

All storage batteries so far in extensive use in telephone systems are
composed of lead plates in a solution of sulphuric acid in water called
the _electrolyte_. In charging, the current tends to oxidize the lead of
one plate and de-oxidize the other. In discharging, the tendency is
toward equilibrium.

The containers, employed in telephone work, for the plates and
electrolyte are either of glass or wood with a lead lining, the glass
jars being used for the smaller sized plates of small capacity cells,
while the lead-lined wooden tanks are employed with the larger capacity
cells. The potential of a cell is slightly over two volts and is
independent of the shape or size of the plates for a given type of
battery. The storage capacity of a cell is determined by the size and
the number of plates. Therefore, by increasing the number of plates and
the areas of their surfaces, the ampere-hour capacity of the cell is
correspondingly increased. The desired potential of the battery is
obtained by connecting the proper number of cells in series.
Storage-battery cells used in telephone work vary from 2 plates having
an area of 12 square inches each, to cells having over 50 plates, each
plate having an area of 240 square inches. The ampere-hour capacity of
these batteries varies from