(3) Switchboard equipment in common-battery systems is much more complex and expensive than local battery
equipment performing comparable functions, and it requires a greater time for installation and maintenance.
(4) The resistance of the loop or line to the common battery telephone station limits the distance over which
talking and signaling currents may be supplied. Sufficient current must flow to assure operation of the following:
(b) The line signal at the central office when the receiver is lifted.
(c) The supervisory relay at the central office when the receiver is hung up.
c. Applications. Because of the greater expense involved in the construction and maintenance of the inside and
outside plant equipment of the stations and much local traffic are concentrated in a small area. In commercial practice,
common-battery systems are used in all cities and large towns; in military applications, they often are used in higher
headquarters. Local-battery systems are much better suited to rural areas where there are relatively few stations scattered
over a large area; but they also are used for field military applications, for they provide better transmission over field wire
than does the common-battery system. In general, common-battery systems have greater application in permanent
Basic Circuits of Common-Battery System.
a. Simple common-battery circuit. The essential feature of a common-battery system is emphasized best by
comparing the simplest possible common-battery system with the simplest possible local-battery system. The essential
difference between the two systems is illustrated in figure 37.
(1) The figure 37A illustrates a very simple local-battery circuit, with hand generators and ringers omitted. The
circuit consists of a transmission line terminated at each end by a telephone set. The telephone sets, A and B, are shown
in elementary form and consist of a transmitter, a receiver, and induction coil, and a local battery.