system shown in Fig. 130. Let us inquire why it is that,
although all of these four pairs of lines are connected with a common
source of energy and are, therefore, all conductively joined, the
stations will be able to communicate in pairs without interference
between the pairs. In other words, why is it that voice currents
originating at Station A will pass only to the receiver at Station B
and not to the receivers at Station C or Station H, for instance? The
reason is that separate supply conductors lead from the points such as
_1_ and _2_ at the junctions of the repeating-coil windings on each
pair of circuits to the battery terminals, and the resistance and
impedance of the battery itself and of the common leads to it are so
small that although the feeble voice currents originating in the pair
of lines connecting Station A and Station B pass through the battery,
they are not able to alter the potential of the battery in any
appreciable degree. As a result, therefore, the supply wires leading
from the common-battery terminals to the points _7_ and _8_, for
instance, cannot be subjected to any variations in potential by virtue
of currents flowing through the battery from the points _1_ and _2_ of
the lines joining Station A and Station B.

[Illustration: MAIN OFFICE, KEYSTONE TELEPHONE COMPANY, PHILADELPHIA,
PA.]

[Illustration: Fig. 138. Common Source for Many Lines]

[Illustration: Fig. 139. Common Source for Many Lines]

Retardation Coil--Single Battery:--In Fig. 139 is shown in similar
manner the current supply from a single battery to four different
pairs of lines, the battery being associated with the lines by the
combined impedance coil and condenser method, which was specifically
dealt with in connection with Fig. 133. The reasons why there will be
no interference between the conversations carried on in the various
pairs of connected lines in this case are the same as those just
considered in connection with the system shown in Fig. 138. The
impedance coils in this case serve to keep the telephone currents
confined to their respective pairs of lines in which they originate,
and this same consideration applies to the system of Fig. 138, for
each of the separate repeating-coil windings of Fig. 138 is in itself
an impedance coil with respect to such currents as might leak away
from one pair of lines on to another.

Retardation Coil--Double Battery:--The arrangement of feeding a number
of pairs of lines acco