c. The two important paths in a common-battery system are the direct current path, for which the two telephone
stations are in parallel with respect to the common battery, and the talking path, which does not include the battery. Filter
networks are used to prevent the battery from short-circuiting the listening station.
d. Common-battery switchboards, like local-battery switchboards, are used to permit efficient connection of any
two telephone stations connected to the switchboard. However, because of the difference in battery supply, there are
differences between the circuits contained in common-battery switchboards and those in local-battery switchboards.
e. Nonmultiple common-battery switchboards are arranged so that each incoming telephone line terminates in
only one line jack of the switchboard. This restricts the use of such switchboards to systems where no more than three
operators are required to handle the traffic. The switchboard contains all the signal, supervisory, and pilot lamps, the line
jacks, the cord and plugs, and the various switches and relays necessary for efficient operation.
f. Common-battery cords have three conductors--tip, ring, and sleeve. Common-battery plugs have
corresponding elements to which the conductors of the cord are connected. Cords normally are plugged; one, the answer
cord, is used in answering a calling station; the other, the call cord, for completing the call to a called station.
g. Simple common-battery jacks have three contacts--tip, ring, and sleeve, corresponding to the three conductors
of a common-battery cord. Cut-off jacks have two or more auxiliary contacts associated with the tip and ring contacts.
The auxiliary contacts either can be made or broken by the movements of the jack springs.
h. Line signals in common-battery systems are line lamps, mounted on the panel of the switchboard above or
below their associated line jacks. Supervisory signals are lamps associated with the cord circuits.
Section II. Major Common-Battery Switchboard Circuits.
55.
Telephone Relays.
a. Application of relays to telephony.
As explained in paragraph 45, one of the important
advantages of common-battery systems over local-battery systems is the provision of automatic signaling
and supervision. This is made feasible by the use if a control device called a relay. The principles of
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