Fig. 133. Dean System
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Dean Substation Arrangement. In marked contrast to the scheme for keeping steady current out of the receiver circuit employed by the Kellogg Company, is that shown in Fig. 133, which has been largely used by the Dean Electric Company, of Elyria, Ohio. The central-office arrangement in this case is that using the split repeating coil, which needs no further description. The substation arrangement, however, is unique and is a beautiful example of what can be done in the way of preventing a flow of current through a path without in any way insulating that path or placing any barrier in the way of the current. It is an example of the prevention of the direct flow of current through the receiver by so arranging the circuits that there will always be an equal potential on each side of it, and, therefore, no tendency for current to flow through it.

In this substation arrangement four coils of wire—1, 2, 3, and 4—are so arranged as to be connected in the circuit of the line, two in series and two in multiple. The current flowing from the battery at the central office, after passing through the transmitter, divides between the two paths containing, respectively, the coils 1 and 3 and the coils 2 and 4. The receiver is connected between the junction of the coils 2 and 4 and that of 1 and 3. The resistances of the coils are so chosen that the drop of potential through the coil 2 will be equal to that through the coil 1, and likewise that through the coil 4 will be equal to that through the coil 3. As a result, the receiver will be connected between two points of equal potential, and no direct current will flow through it. How, then, do voice currents find their way through the receiver, as they evidently must, if the circuit is to fulfill any useful function? The coils 2 and 3 are made to have high impedance, while 1 and 4 are so wound as to be non-inductive and, therefore, offer no impedance save that of their ohmic resistance. What is true, therefore, of direct currents does not hold for voice currents, and as a result, the voice currents, instead of taking the divided path which the direct currents pursued, are debarred from the coils 2 and 3 by their high impedance and thus pass through the non-inductive coil 1, the receiver, and the non-inductive coil 4.

This circuit employs a Wheatstone-bridge arrangement, adjusted to a state of balance with respect to direct currents, such currents being excluded from the receiver, not because the receiver circuit is in any sense opaque to such direct currents, but because there is no difference of potential between the terminals of the receiver circuit, and, therefore, no tendency for current to flow through the receiver. In order that fluctuating currents may not, for the same reason, be caused to pass by, rather than through, the receiver circuit, the diametrically-opposed arms of the Wheatstone bridge are made to possess, in large degree, self-induction, thereby giving these two arms a high impedance to fluctuating currents. The conditions which exist for direct currents do not, therefore, exist for fluctuating currents, and it is this distinction which allows alternating currents to pass through the receiver and at the same time excludes direct currents therefrom.

In practice, the coils 1, 2, 3, and 4 of the Dean substation circuit are wound on the same core, but coils 1 and 4—the non-inductive ones—are wound by doubling the wire back on itself so as to neutralize their self-induction.

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.

Fig. 134. Stromberg-Carlson System
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