rformance of a task that would require but a few minutes' time,
may consume some hours on the part of the employes in getting there and
back.

Common Battery. _Advantages._ It would be more economical if all of
the current for the subscribers' transmitters could be supplied from a
single comparatively efficient generating source instead of from a
multitude of inefficient small sources scattered throughout the
community served by the exchange. The advantage of such centralization
lies not only in more economic generating means, but also in having
the common source of current located at one place, where it may be
cared for with a minimum amount of expense. Such considerations have
resulted in the so-called "common-battery system," wherein the current
for all the subscribers' transmitters is furnished from a source
located at the central office.

Where such a method of supplying current is practiced, the result has
also been, in nearly all cases, the doing away with the subscriber's
magneto generators, relying on the central-office source of current to
furnish the energy for enabling the subscriber to signal the operator.
Such systems, therefore, concentrate all of the sources of energy at
the central office and for that reason they are frequently referred to
as central-energy systems.

NOTE. In this chapter the central-energy or common-battery system
will be considered only in so far as the supply of current for
energizing the subscribers' transmitters is concerned, the
discussion of the action of signaling being reserved for
subsequent chapters.

_Series Battery._ If but a single pair of lines had to be considered,
the arrangement shown in Fig. 128 might be employed. In this the
battery is located at the central office and placed in series with the
two grounded lines leading from the central office to the two
subscribers' stations. The voltage of this battery is made sufficient
to furnish the required current over the resistance of the entire line
circuit with its included instruments. Obviously, changes in
resistance in the transmitter at Station A will affect the flow of
current in the entire line and the fluctuations resulting from the
vibration of the transmitter diaphragm will, therefore, reproduce
these sounds in the receiver at Station B, as well as in that at
Station A.

[Illustration: Fig. 128. Battery in Series with Two Lines]

An exactly similar arrangement applied to a metalli