accomplished by the use of filter networks (fig. 38). The actual filter networks used in common-battery circuits are either
retardation-coil or repeating-coil arrangements. Although the filters present a relatively high impedance to alternating
currents, their dc resistance is so low that they do not affect materially the direct current from the battery to the telephone
d. Practical common-battery circuits. In the following paragraphs, the simplified circuit explained above will be
used as a starting point to outline the need for the various circuits and components used in practical systems. For
example, the simplified circuits of figures 37 and 38 show only two telephone stations, and the common battery is bridged
across the line. In practice, the system would include many more two stations, and any two stations would be connected
through one or more switchboards or central offices. Also, in practice, the common battery is located at a central office,
and is connected through a power distributing panel to the switchboard to the telephone stations when they are in use.
Associated with the switchboard are various circuits required to develop the advantages of efficient operation of which the
common-battery system is capable. The switchboard may be regarded as the heart of the system. Although the
switchboard serves the same basic function in the common-battery system as in the local-battery system, the common-
battery switchboard differs in many of its details from the local-battery switchboards discussed in chapter 4. Therefore,
before proceeding with a full discussion of practical common-battery circuits, the common-battery switchboard will be
(1) As in the case of the local-battery switchboard, the main function of the common-battery switchboard is to
permit efficient connection of any two telephone stations connected to it. The switchboard also must provide means for
supervision of calls, and must enable the operator to ring the called station. Most ringing is performed by ringing
machines and circuits similar to those of a local-battery system.
(2) The efficient operation of a common-battery system is made possible by the various circuits contained in the
switchboard. They include line circuits and cord circuits of various kinds, trunk circuits, ringing circuits, supervision
circuits, and various auxiliary circuits.