eed and increase its
counter-electromotive force without overheating. The accepted type has
means for opening the driving circuit automatically in case its voltage
should fall, thus preventing a temporary interruption of driving current
from damaging the motor armature on its return to normal voltage.

[Illustration: Fig. 417. Power-Plant Circuits]

_Meters._ The meters usually are voltmeters and ammeters, the former
being adapted to read the several voltages of direct currents in the
power plant. An important one to be known is the voltage of the
generator before beginning a battery charge, so that the generator may
not be thrown on the storage battery while generating a voltage less
than that of the battery. If this were done, the battery would discharge
through the generator armature. The voltmeter enables the voltage of the
charging generator to be kept above that of the battery, as the latter
rises during charge. It enables the performance of several cells of the
battery to be observed. A convenient way is to connect the terminals of
the several cells to jacks on the power board and to terminate the
voltmeter in a plug.

The ammeter, with suitable connections, enables the battery-charge rate
to be kept normal and the battery discharge to be observed. In order to
economize power, it is best to charge the battery during the hours of
heavy load. The generator output then divides, the switchboard taking
what the load requires, the battery receiving the remainder.

In systems requiring the terminal voltage of the equipment to be kept
constant within close limits, either it is necessary to use two
batteries--never drawing current from a battery during charge--or to
provide means of compensating for the rise of voltage while the battery
is under charge. The latter is the more modern method and is done either
by using fewer cells when the voltage per cell is higher or by inserting
counter-electromotive force cells in the discharge leads, opposing the
discharge by more or fewer cells as the voltage of the battery is
higher or lower. In either method, switches on the power board enable
the insertion and removal of the necessary end cells or
counter-electromotive force cells.

_Protective Devices._ The protective devices required on a power board
are principally _circuit-breakers_ and _fuses_. Circuit-breakers are
adapted to open motor and generator circuits when their currents are too
great, too small, or in the wrong d