bscribers' station circuits, is shown in
Fig. 134, this being a simplified representation of the circuits
commonly employed by the Stromberg-Carlson Telephone Manufacturing
Company. The battery feed at the central office differs only from that
shown in Fig. 132, in that a single battery rather than two batteries
is used, the current being supplied to one of the lines through the
impedance coils _1_ and _2_, and to the other line through the
impedance coils _3_ and _4_; condensers _5_ and _6_ serve conductively
to isolate the two lines. At the subscriber's station the line circuit
passes through the secondary of an induction coil and the transmitter.
The receiver is kept entirely in a local circuit so that there is no
tendency for direct current to flow through it, but it is receptive to
voice currents through the electromagnetic induction between the
primary and the secondary of the induction coil.

[Illustration: Fig. 134. Stromberg-Carlson System]

[Illustration: Fig. 135. North Electric Company System]

_North._ Another arrangement of central-office battery feed is
employed by the North Electric Company, and is shown in Fig. 135. In
this two batteries are used which supply current respectively to the
two connected lines, condensers being employed to conductively isolate
the lines. This differs from the Kellogg arrangement shown in Fig. 132
in that the two coils _1_ and _2_ are wound on the same core, while
the coils _3_ and _4_ are wound together upon another core. In this
case, in order that the inductive action of one of the coils may not
neutralize that of the other coil on the same core, the two coils are
wound in such relative direction that their magnetizing influence
will always be cumulative rather than differential.

The central-office arrangements discussed in Figs. 130 to 135,
inclusive, are those which are in principal use in commercial practice
in common-battery exchanges.

_Current Supply over Limbs of Line in Parallel._ As indicating further
interesting possibilities in the method of supplying current from a
common source to a number of substations, several other systems will
be briefly referred to as being of interest, although these have not
gone into wide commercial use. The system shown in Fig. 136 is one
proposed by Dean in the early days of common-battery working, and this
arrangement was put into actual service and gave satisfactory results,
but was afterwards supplanted by the Bell equipme