b.
Operation.
(1)
is approximately the same as that of the sidetone circuit
described in paragraph 47, both the direct current from
the central office battery and the voice-frequency
currents corresponding to the sound waves striking the
diaphragm of the transmitter are similar to those in the
sidetone circuit. In this circuit, however, because of the
different location of the transmitter with respect to the
receiver, all of the voice current originating in the
transmitter flows in the primary winding of the induction
coil, and only by induction in the receiver circuit. The
through the circuit comprised of the ringer and capacitor
FIGURE 47. Common-Battery Sidetone
or the ringer, secondary winding, and receiver.
Reduction Circuit.
(2) The current fluctuations in the primary winding of the induction coil induce a corresponding alternating emf
in the secondary winding. The primary and secondary currents are in phase because of the transformer connections, but
little reinforcement of current occurs because there is no impedance common to primary and secondary circuits. As a
result, sidetone is reduced.
(3) Although the sidetone-reduction circuit is effective in reducing the amount of sidetone, it lacks the desirable
feature of the sidetone circuit--that of causing a higher voltage to be impressed on the line by booster action. For this
reason, transmission with the sidetone-reduction circuit is less efficient than with the booster circuit discussed in the
preceding paragraph.
101