battery to two telephone sets. It can be seen that, as far as direct current is concerned, the two telephone sets and their
associated telephone lines are in parallel with respect to the common battery. If several telephone circuits were connected
across the single battery, the direct current which the battery would be required to furnish easily could reach extremely
large values. For this reason, the common battery in a common-battery system usually is connected permanently to a
battery charger. The function and operation of a battery charger will be explained more fully in a later chapter.
c. Talking path in simple common-battery circuit.
(1) It has been shown (ch. 3) that the operation of a telephone transmitter is based on the principle that sound
waves, striking the diaphragm of the transmitter, produce corresponding vibrations of the diaphragm. The varying
pressure of the diaphragm varies the resistance of the carbon granules, and causes the current to fluctuate about its normal
steady value. The resulting pulsating direct current therefore can be considered to consist of a steady direct current on
which is superimposed a voice-frequency alternating current. The paths of the dc and ac components of the transmitter
current are not necessarily the same.
(2) The talking path in the simplified common-battery circuit is shown in figure 37. At first glance it would
appear that the two telephone sets are in series so far as the talking path is concerned. However, the common battery now
must be considered in parallel with the receiver and transmitter of the listening station. Thus, if the person at station A is
talking into his transmitter, the voice-frequency currents originating at transmitter A flow through the line until they reach
the point where the common battery is bridged across the line. Since the impedance of the battery to the voice-frequency
currents is much lower than that of the transmitter-receiver combination of station B, the major portion of the voice-
frequency currents will flow through the battery leg. The same situation arises when the voice-frequency currents
originate at the transmitter of station B. The battery, by shunting the receiver branch of the listening station, greatly
reduces the magnitude of the voice-frequency currents flowing in the receiver.
(3) In order to prevent the common battery from shunting the receiver branch of the listening station,
some component must be inserted in series with the battery in the battery leg to voice-frequency currents. This is