Their relation to each other and to the other components of the circuit differ in a common-battery system.
b. Operation of basic circuit. The basic simplified circuit of a common-battery telephone set (fig. 45) can be
considered conveniently in three parts: the primary, or transmitter circuit, the secondary, or receiver circuit, and the
ringing circuit.
(1) The primary circuit of a telephone set, shown in the figure, consists of the transmitter in series with the
primary winding of induction coil I, hookswitch H, and line terminals L1 and L2. The hookswitch, as explained in (3)
below, closes this circuit whenever the receiver is lifted from its hook or cradle, as when a user lifts it to make or answer a
call. The battery at the central office may be considered connected effectively to terminals L1 and L2, on answering a
call, by means of the cord circuit, line jack, and telephone line. When the hookswitch contacts are closed, direct current
from the common battery flows through the primary winding of the induction coil and the transmitter. Words spoken into
the transmitter cause a pulsating direct current to flow through the primary winding of the induction coil and over the line.
A similar telephone set, at the calling station, is connected to this set, and the similar pulsating direct current flows
through the primary winding of its induction coil. By transformer action, an alternating current flows in the secondary
winding of the induction coil of the called set and in its receiver, where the sound waves striking the calling transmitter
diaphragm are reproduced.
(2) The secondary circuit consists simply of the secondary winding of the induction coil in series with the
receiver. It has been explained above that the pulsating direct current in the primary winding of the induction coil of the
called set causes an alternating current to flow in the secondary winding and the receiver of the called set. However, by
the same transformer action, the pulsating direct current in the primary winding of the induction coil of the calling set also
causes a corresponding alternating current to flow in the secondary winding and the receiver of the calling set. This
results in the production of an appreciable amount of sidetone so that the speaker hears his own voice in his own receiver.
This undesirable effect is minimized by the use of circuits described below (pars. 51 through 53).
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