b. Operation of circuit.
(1) The operation of the antisidetone circuit shown in A can be understood by referring to its equivalent circuit
for transmitting, in B. Voice currents originating in the transmitter flow in the primary coil, P, inducing voltages in both
secondary windings. These voltages are represented in B by the ac generators--E1 for the voltage induced in winding S,
and E2 for winding B. Voltage E1 causes a current to flow through the parallel combination of the receiver shunted by
winding B and resistor N, through the transmitter and back to winding S through the capacitor (omitted from B, since it is
practically a short circuit to voice-frequency currents). A portion of this current, of course, flows through the receiver in
the direction shown by the unbroken arrows. At the same time, voltage E2 causes a current consisting of the receiver,
resistor N, and winding B, in the direction of the broken-line arrows. The two currents through the receiver flow in
opposite directions. By proper choice of the resistance of resistor N, the current caused by E2 is made exactly equal to the
current through the receiver caused by E1. The two currents thus cancel, or balance, each other through the operation of
(2) Actually, the circuit shown in figure 48 gives a perfect balance at only one frequency in the voice-frequency
range. However, sidetone is reduced greatly over the entire telephone frequency range with this circuit. It is only one of
several antisidetone circuits that have been developed in recent years.
(3) In receiving from a distant station, the operation of the receiver circuit in figure 48 is similar to that in figure
45, and is not affected by the antisidetone feature.
Summary of Basic Principles and Components.
a. Common-battery systems use a centrally located storage battery in place of the individual dry cells at the
telephone stations in local-battery systems. The common battery is actually an auxiliary source of power, the main source
being either a motor-generator set or a rectifier system.
b. The common battery gives a common-battery system certain advantages over a local-battery system,
including automatic signaling and simpler supervision and maintenance. However, they can be used efficiently only
where the traffic is heavy enough to warrant the relatively high construction and installation costs.