b. Path of simplex-circuit current.
(1) At the same time that this conversation is taking place, a person at telephone T3 may communicate with
telephone T4 (fig. 68). The voice current at the center terminal of the secondary of RC1 divides equally into the two
windings of the secondary of RC1, as shown by the broken-line arrows. Since equal currents flow in opposite directions
through the two windings, no emf is induced in the primary of RC1, and no voice current flows in the receiver of T1. The
conversation originating in T3 therefore is not heard in T1.
(2) The voice current flows through both wires of the metallic line, however, as indicated by the broken arrows.
The current flowing up through the top winding of the secondary of RC1 flows through the upper line and through the top
winding of the secondary of RC2. Similarly, the current flowing down through the bottom winding of the secondary of
RC1 flows through the lower line and through the bottom winding of the secondary of RC2. These two currents join at
the center terminal of RC2 and flow through the receiver of telephone T4. The path is completed through ground, as
indicated by the dashed line. Just as the two windings of the secondary of RC1 induce neutralizing (canceling) emf 's in
the primary of RC1, the two currents in the windings of the secondary of RC2 induce neutralizing emf 's in the primary of
RC2 is zero, no current resulting from the conversation between T3 and T4 flows into the receiver of T2. Consequently, a
means is provided of carrying on two independent conversations without mutual interference on a single metallic circuit.
c. Line balancing with simplex circuit.
(1) The operation of a simplex circuit takes place without mutual interference only if the repeating coils have
identical windings, and if the wires of the metallic line have the same impedance. It may be assumed that proper design
and manufacture of the repeating coils eliminates any unbalance resulting from the coils. However, interference caused
by different impedances in the two wires of the metallic circuit may be troublesome. The main cause of unbalance in the
lines of field-wire circuits is caused by poor splices, which may introduce a high resistance into one side of the circuit.
Another cause of unbalance is improperly taped splices and worn and damaged insulation, which may result in excessive
leakage from one side of the circuit to ground, particularly when the wires are wet.