This impedance-coil method seems to present the advantage of greater simplicity over the repeating-coil method shown in Fig. 130, and so far as talking efficiency is concerned, there is little to choose between the two.  The repeating-coil method, however, has the advantage over this impedance-coil method, because by it the two lines are practically divided except by the inductive connection between the two windings, and as a result an unbalanced condition of one of the connected lines is not as likely to produce an unbalanced condition in the other as where the two lines are connected straight through, as with the impedance-coil method.  The substation arrangement of Fig. 131 is the same as that of Fig. 130.

[Illustration:  Fig. 132.  Double-Battery Kellogg System]

Double Battery with Impedance Coils. A modification of the impedance-coil method is used in all of the central-office work of the Kellogg Switchboard and Supply Company.  This employs a combination of impedance coils and condensers, and in effect isolates the lines conductively from each other as completely as the repeating-coil method.  It is characteristic of all the Kellogg common-battery systems that they employ two batteries instead of one, one of these being connected in all cases with the calling line of a pair of connected lines and the other in all cases with the called line.  As shown in Fig. 132, the left-hand battery is connected with the line leading to Station A through the impedance coils 1 and 2.  Likewise, the right-hand battery is connected to the line of Station B through the impedance coils 3 and 4.  These four impedance coils are wound on separate cores and do not have any inductive relation whatsoever with each other.  Condensers 5 and 6 are employed to completely isolate the lines conductively.  Current from the left-hand battery, therefore, passes only to Station A, and current from the right-hand battery to Station B. Whenever the transmitter at Station A is actuated the undulations of current which it produces in the line cause a varying difference of potential across the outside terminals of the two impedance coils 1 and 2.  This means that the two left-hand terminals of condensers 5 and 6 are subjected to a varying difference of potential and these, of course, by electrostatic induction, cause the right-hand terminals of these condensers to be subject to a correspondingly varying difference of potential.  From this it follows that alternating currents will be impressed upon the right-hand line and these will affect the receiver at Station B.

A rough way of expressing the action of this circuit is to consider it in the same light as that of the impedance-coil circuit shown in Fig. 131, and to consider that the voice currents originating in one line are prevented from passing through the bridge paths at the central office on account of the impedance, and are, therefore, forced to continue on the line, being allowed to pass readily by the condensers in series between the two lines.