(2) For purposes of comparison, an equally simple common-battery circuit also is illustrated. In this diagram, a
common battery is shown connected across the telephone line, replacing the individual local batteries. Each telephone set
consists of a transmitter, a receiver and an induction coil. Reference to the circuits will indicate one outstanding
difference: In the local-battery circuit, the receiver and the secondary, S, winding of the induction coil are wired in series
and connected across the line; in the common-battery circuit, the transmitter and the primary, P, winding of the induction
coil are wired in series and connected across the line. A later paragraph describes more in detail the practical circuits used
in common-battery systems.
b. Direct-current paths of simple common-battery circuit.
As explained in chapter 3, direct
current flows through the transmitter when it is operating. In the simple local-battery circuit, direct current is
furnished by the local battery in series with the transmitter and primary winding of the induction coil at
each station.
In the common-battery circuit, however, direct current is furnished to both transmitters
by the common battery, as shown in figure 37. The figure illustrates the direct-current path from the common
FIGURE 37. Comparison Between Simple
Common Battery Telephone
Circuits
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