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 paths by virtue of the impedance.

[Illustration:  Fig. 137.  Current Supply over Parallel Limbs of Line]

An objection to this form of current supply and to other similar forms, wherein the transmitter current is fed over the two sides of the line in multiple with a ground return, is that the ground-return circuit formed by the two sides of the line in multiple is subject to inductive disturbances from other lines in the same way as an ordinary grounded line is subject to inductive disturbance.  The current-supply circuit is thus subject to external disturbances and such disturbances find their way into the metallic circuit and, therefore, through the instruments by means of the electromagnetic induction between the primary and the secondary coils at the substations.