Another interesting method of current supply from a central-office battery is shown in Fig. 137.  This, like the circuit just considered, feeds the energy to the subscriber’s station over the two sides of the line in multiple with a ground return.  In this case, however, a local circuit is provided at the substation, in which is placed a storage battery 1 and the primary 2 of an induction coil, together with the transmitter.  The idea in this is that the current supply from the central office will pass through the storage battery and charge it.  Upon the use of the transmitter, this storage battery acts to supply current to the local circuit containing the transmitter and the primary coil 2 in exactly the same manner as in a local battery system.  The fluctuating current so produced by the action of the transmitter in this local circuit acts on the secondary winding 3 of the induction coil, and produces therein alternating currents which pass to the central office and are in turn repeated to the distant station.

Supply Many Lines from Common Source. We come now to the consideration of the arrangement by which a single battery may be made to supply current at the central office to a large number of pairs of connected lines simultaneously.  Up to this point in this discussion it has been shown only how each battery served a single pair of connected lines and no others.

Repeating Coil:—­In Fig. 138 is shown how a single battery supplies current simultaneously to four different pairs of lines, the lines of each pair being connected for conversation.  It is seen that the pairs of lines shown in this figure are arranged in each case in accordance with the system shown in Fig. 130.  Let us inquire why it is that, although all of these four pairs of lines are connected with a common source of energy and are, therefore, all conductively joined, the stations will be able to communicate in pairs without interference between the pairs.  In other words, why is it that voice currents originating at Station A will pass only to the receiver at Station B and not to the receivers at Station C or Station H, for instance?  The reason is that separate supply conductors lead from the points such as 1 and 2 at the junctions of the repeating-coil windings on each pair of circuits to the battery terminals, and the resistance and impedance of the battery itself and of the common leads to it are so small that although the feeble voice currents originating in the pair of lines connecting Station A and Station B pass through the battery, they are not able to alter the potential of the battery in any appreciable degree.  As a result, therefore, the supply wires leading from the common-battery terminals to the points 7 and 8, for instance, cannot be subjected to any variations in potential by virtue of currents flowing through the battery from the points 1 and 2 of the lines joining Station A and Station B.