(4) The filters in series with the battery leg prevent the flow of talking current through the battery. Besides
preventing the battery from short-circuiting the talking current to the listening station, they prevent the various cord
circuits from interfering with each other. For example, suppose a conversation is in progress between telephones 1003
and 1004 by way of cord circuit 2, as in B, in addition to the conversation between telephones 1001 and 1002. If no filters
were present in the cord circuits, talking current originating at any of the four stations theoretically could flow in the
receivers of all of the other stations. This, of course, would make impossible a private conversation between only two
(5) With the circuit arrangement shown in B, however, it is possible for any two stations to have a private
conversation, without interference from any other conversations that might be in progress by way of the other cord
circuits. For example, if station 1001 wishes to converse with station 1003, the answer cord would be connected to the
line jack of station 1001, and the calling cord would be connected to the line jack of station 1003. The cord circuits in the
switchboard therefore provide a rapid and efficient means of interconnecting the various lines terminating on the
switchboard, each cord circuit accommodating a conversation between any two stations.
(6) Figure 39 shows that, although the switchboard has 100 line jacks and lamps accommodating 100 telephone
stations, it has only 15 cord circuits and an additional answer cord (located at the extreme right of the plugshelf). This
enables the switchboard to handle up to 15 individual calls at any one time, so that this particular switchboard would be
used in a common-battery system where the traffic normally would be not more than 15 calls at any one time. The
additional answer cord (not found on all switchboards) is used to notify a calling telephone that all the regular cord
circuits are busy when that situation arises.
Basic Components of Common-Battery Cord and Line Circuits.
a. Common-battery cords and plugs.
(1) The functions of the cords and plugs of the cord circuits in common-battery systems, as explained above, are
essentially similar to those associated with local-battery systems. In addition to providing connections between a calling
and a called telephone for talking and signaling, however, common-battery cords and plugs provide a means of automatic
supervision through their respective sleeve conductors and contacts.