ystem, therefore, lends
itself naturally to the purposes of automatic signaling, since it is
only necessary to place at the central office a device in the circuit of
each line that will be responsive to the current which flows from the
central battery when the subscriber removes his receiver from its hook.
It is thus that the subscriber is enabled automatically to signal the
central office when he desires a connection; and as will be shown, it is
by the same sort of means, associated with the cord circuits used in
connecting his line with some other line, that the operator is
automatically notified when a disconnection is desired, the cessation of
current through the subscriber's line when he hangs up his receiver
being made to actuate certain responsive devices which are associated
with the cord at that time connected with his line, and which convey the
proper disconnect signal to the operator.

Concentration of sources of energy into a single large unit, the
simplification of the subscriber's station equipment, and the ready
adaptability to automatic signaling from the subscriber to the central
office are, therefore, the reasons for the existence of the
common-battery system.

=Common Battery vs. Magneto.= It must not be supposed, however, that the
common-battery system always has advantages over the magneto system, and
that it is superior to the magneto or local-battery system for all
purposes. It is the outward attractiveness of the common-battery system
and the arguments in its favor, so readily made by over-zealous
salesmen, that has led, in many cases, to the adoption of this system
when the magneto system would better have served the purpose of utility
and economy.

To say the least, the telephone transmission to be had from
common-battery systems is no better than that to be had from
local-battery systems, and as a rule, assuming equality in other
respects, it is not as good. It is perhaps true, however, that under
average conditions common-battery transmission is somewhat better,
because whereas the local batteries at the subscribers' stations in the
local-battery system are not likely to be in uniformly first-class
condition, the battery in a common-battery system will be kept up to its
full voltage except under the grossest neglect.

The places in which the magneto, or local-battery, system is to be
preferred to the common-battery system, in the opinion of the writers,
are to be found in the small rural