ion.

=Primary Sources.= Most telephone power plants consume commercial
electric power and deliver special electric current. Usually some
translating device, such as a motor-generator or a mercury-arc
rectifier, is employed to transform the commercial current into the
specialized current required for the immediate uses of the exchange.

_Charging from Direct-Current Mains._ In some cases commercial direct
current is used to charge the storage batteries without the intervention
of the translating devices, resistances being used in series with the
battery to regulate the amount of current. Commercial direct current
usually is available at pressures from 110 volts and upward, while
telephone power plants contain storage batteries rarely of pressures
higher than 50 volts. To charge a 50-volt storage battery direct from
110-volt mains results in the loss of about half the energy purchased,
this lost energy being set free in the form of heat generated in the
resistance devices. Notwithstanding this, it is sometimes economical to
charge directly from the commercial direct-current power mains, but only
in small offices where the total amount of current consumed is not large
and where the greatest simplicity in equipment is desirable. It is
better, however, in nearly all cases, to convert the purchased power
from the received voltage to the required voltage by some form of
translating device, such as a rotary converter or a mercury-arc
rectifier.

_Rotary Converters._ Broadly speaking, a rotary converter consists of a
motor adapted to the voltage and kind of current received, mechanically
coupled to a generator adapted to produce current of the required kind
and voltage. The harmonic ringing machine shown in Fig. 413 is an
example of this, this particular one being adapted to receive direct
current at ordinary commercial pressure and to deliver four different
alternating currents of suitable pressures and frequencies. It is to be
understood, however, that the conversion may be from direct current to
direct current, from alternating to direct, or from direct to
alternating. Such a device where the motor is a separate and distinct
machine from the generator or generators is called a _motor-generator_.
It is usual to connect the motors and the generators together directly
by a coupling having some flexibility, as shown in Fig. 413, so as to
prevent undue friction in the bearings.

[Illustration: THE POWER AND WIRE CHIEF'S ROOM