mple device.

The supply of current to private branch exchanges over wires leading to
the central-office battery has the disadvantage of requiring one or
several pairs of wires in the cables carrying the trunk wires. No
special wires are run, regular pairs in the paper insulated line or
trunk cables being admirably suited for the purpose. Sufficient
conductivity may be provided by placing several such pairs in multiple.

If the amount of current required by the private exchange warrants it,
pairs of charging wires from the central office may be fewer if a
battery is charged over them than if they are used direct to the bus
bars of the private exchange switchboard. If they are used in the latter
way, and this is simpler for reasons of maintenance, some means must be
provided to prevent the considerable resistance of the supply wires from
introducing cross-talk into the circuit of the private exchange. This is
accomplished by bridging a considerable capacity across the supply pairs
at the private exchange--ten to twelve microfarads usually suffice. This
point has already been referred to and illustrated in connection with
Fig. 141.

The number of pairs of wires, or, in other words, the amount of copper
in the battery lead between the central office and the private
branch-exchange switchboard needs to be properly determined not only to
eliminate cross-talk when the proper condensers are used with them, but
to furnish the proper difference of potential at the private exchange
bus bars, so that the line and supervisory signals will receive the
proper current. It is a convenience in installing and maintaining
private exchange switchboards of this kind to prepare tables showing the
number of pairs of No. 19 gauge and No. 22 gauge wires required for a
private exchange at a given distance from its central office and of a
probable amount of traffic. The traffic may be expressed in the maximum
number of pairs of cords which will be in use at one time. With this
fact and the distance, the number of pairs of wires required may be
determined.

=Ringing Current.= The ringing current may be provided in two ways: over
pairs of wires from the city-office ringing machines or by means of a
local hand generator, or both. A key should enable either of these
sources of ringing current to be chosen at will.

=Marking of Apparatus.= All apparatus should be marked with permanent
and clear labels. That private exchange switchboard is best at