(3) Because the battery of a common-battery system is located at the central office, and is therefore always in the
circuit, some means must be provided for making the circuit when a conversation is to be started, for keeping it closed
while the telephone sets are in use, and for breaking or when the telephone conversation is over. This function is
performed by a hookswitch in series with the primary circuit (fig. 45). The receiver in a common-battery set hangs on the
hook of the hookswitch when not in use, and its weight keeps the hookswitch contacts normally open. Removal of the
receiver causes the hookswitch contacts to close. Unlike the handswitch of the local-battery switch set, which may be the
type that is pushed for talking and released for listening, the hookswitch contacts of a common-battery set remain closed
during the entire conversation.
c. Ringer circuit. The ringer circuit of the basic common-battery telephone set consists of a ringer in series with
a capacitor, usually of .5- f capacity. The combination is connected directly across the line, between terminals L1 and L2.
The capacitor, in the common-battery set, blocks the flow of direct current through the ringer, but permits the flow of
ringing current. The ringer does not shunt the primary circuit of the set appreciably so far as talking current is concerned,
because the impedance of the ringer path to voice frequencies is much greater than that of the primary winding of the
Common-Battery Sidetone (Booster) Circuit.
a. Arrangement. The basic circuit of a common-battery telephone set has been found (par. 50b) to produce
considerable sidetone in either receiver. A more efficient circuit which, although it also produces sidetone, represents a
considerable improvement over the basic circuit of figure 45 is the so-called booster circuit (fig. 46). It contains the same
components as the telephone set shown in figure 45, but they are arranged differently. In this circuit, the receiver and the
secondary winding of the induction coil are connected in series between the junction of the ringer and capacitor at A and
the upper hookswitch contact, which is electrically the same as B when the hookswitch contacts are closed. Although this
arrangement permits some direct current to flow through the ringer and the receiver, the high resistance of these
components makes the amount small.