nt operating under
the system shown in Fig. 130, which became standardized by that
company. In this the current from the common battery at the central
office is not fed over the two line wires in series, but in multiple,
using a ground return from the subscriber's station to the central
office. Across the metallic circuit formed by two connected lines
there is bridged, at the central office, an impedance coil _1_, and
between the center point of this impedance coil and the ground is
connected the common battery. At the subscriber's station is placed an
impedance coil _2_, also bridged across the two limbs of the line, and
between the center point of this impedance coil and the ground is
connected the transmitter, which is shunted by the primary winding of
an induction coil. Connected between the two limbs of the line at the
substation there is also the receiver and the secondary of an
induction coil in series.

[Illustration: Fig. 136. Current Supply over Parallel Limbs of Line]

The action of this circuit at first seems a little complex, but if
taken step by step may readily be understood. The transmitter supply
circuit may be traced from the central-office battery through the two
halves of the impedance coil _1_ in multiple; thence over the two
limbs of the line in multiple to Station A, for instance; thence in
multiple through the two halves of impedance coil _2_, to the center
point of that coil; thence through the two paths offered respectively
by the primary of the induction coil and by the transmitter; then to
ground and back to the other pole of the central-office battery. By
this circuit the transmitter at the substation is supplied with
current.

Variations in the resistance of the transmitter when in action, cause
complementary variations in the supply current flowing through the
primary of the induction coil. These variations induce similar
alternating currents in the secondary of this coil, which is in series
in the line circuit. The currents, so induced in this secondary, flow
in series through one side of the line to the distant station; thence
through the secondary and the receiver at that station to the other
side of the line and back through that side of the line to the
receiver. These currents are not permitted to pass through the bridged
paths across the metallic circuit that are offered by the impedance
coils _1_ and _2_, because they are voice currents and are, therefore,
debarred from these pa