her, a primary battery which furnishes current to
energize the transmitter. In such a system, therefore, each telephone
has its own power plant. The term power plant, however, as commonly
employed in telephone work, refers more particularly to the organization
of devices at the central office for furnishing the required kinds of
current, and it is to power plants in this sense that this chapter is
devoted.

_Magneto Systems._ If magneto lines be connected to a switchboard, the
current for throwing the drop at the switchboard is furnished by the
subscriber's generator, and the current for energizing the subscriber's
transmitter is furnished by the local battery at his station; but
sources of current must be provided for enabling the central-office
operator to signal or talk to the subscribers. These are about the only
needs for which current must be furnished in an ordinary magneto central
office. If a multiple board is employed, direct current is also needed
for the purpose of the busy test and also for operating the drop
restoring circuits, if the electrical method of restoring the drops is
employed.

_Common-Battery Systems._ In common-battery systems the requirements are
very much more extensive. The subscribers' telephones have no power
plants of their own, but are provided with a common source of direct
current located at the central office for supplying the talking current,
and for operating the central-office signals, and the operators are
provided with one or more common sources of alternating or pulsating
current for ringing the subscribers' bells. Common-battery equipment
requires the use of currents of different kinds for a greater number of
auxiliary purposes than does magneto equipment. These facts make the
power plant in a common-battery office much more important than in a
magneto office.

=Operators' Transmitter Supply.= In a small magneto exchange, the
transmitter current may be had from primary batteries, a separate
battery being employed for each operator's set. When there are more than
three or four operators, however, it is usual, even in magneto offices,
to obtain the transmitter current from a common storage battery. A
storage battery has the fortunate quality of very low internal
resistance, therefore a number of operators' transmitters may be
actuated by one source without introducing cross-talk. In other words, a
storage battery is a current-furnishing device of good regulation, the
variati