Kellogg Substation Arrangement. An interesting form of substation circuit which is employed by the Kellogg Company in all of its common-battery telephones is shown in Fig. 132.  In passing, it may be well to state that almost any of the substation circuits shown in this chapter are capable of working with any of the central-office circuits.  The different ones are shown for the purpose of giving a knowledge of the various substation circuits that are employed, and, as far as possible, to associate them with the particular central-office arrangements with which they are commonly used.

In this Kellogg substation arrangement the line circuit passes first through the transmitter and then divides, one branch passing through an impedance coil 7 and the other through the receiver and the condenser 8, in series.  The steady current from the central-office battery finds ready path through the transmitter and the impedance coil, but is prevented from passing through the receiver by the barrier set up by the condenser 8.  Voice currents, however, coming over the line to the station, find ready path through the receiver and the condenser but are barred from passing through the impedance coil by virtue of its high impedance.

In considering the action of the station as a transmitting station, the variations set up by the transmitter pass through the condenser and the receiver at the same station, while the steady current which supplies the transmitter passes through the impedance coil.  Impedance coils used for this purpose are made of low ohmic resistance but of a comparatively great number of turns, and, therefore, present a good path for steady currents and a difficult path for voice currents.  This divided circuit arrangement employed by the Kellogg Company is one of the very simple ways of eliminating direct currents from the receiver path, at the same time allowing the free passage of voice currents.

[Illustration:  Fig. 133.  Dean System]

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.