Stromberg-Carlson. Another modification of the central-office arrangement and also of the subscribers’ 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 equipment operating under the system shown in Fig. 130, which became standardized by that company.  In this the current from the common battery at the central office is not fed over the two line wires in series, but in multiple, using a ground return from the subscriber’s station to the central office.  Across the metallic circuit formed by