there is a drop of potential across the transmitter terminals due to the flow of steady current.  This means that the upper terminal of the condenser will be charged to the same potential as the upper terminal of the transmitter, while the lower terminal of the condenser will be of the same potential as the lower terminal of the transmitter.  When, now, the transmitter varies its resistance, a variation in the potential across its terminals will occur; and as a result, a variation in potential across the terminals of the condenser will occur, and this means that alternating currents will flow through the primary winding of the induction coil.  The transmitter, therefore, by its action, causes alternating currents to flow through the primary of this induction coil and it causes, by direct action on the circuit of the line, fluctuations in the steady current flowing in the line.  The alternating currents flowing in the primary of the coil induce currents in the secondary of the coil which supplement and augment the fluctuations produced by the direct action of the transmitter.  This circuit may be looked at, therefore, in the light of combining the direct action which the transmitter produces in the current in the line with the action which the transmitter produces in the local circuit containing the primary of the induction coil, this action being repeated in the line circuit through the secondary of the induction coil.

The receiver in this circuit is placed in the local circuit, and is thus not traversed by the steady currents flowing in the line.  There is thus no necessity for poling it.  This circuit is very efficient, but is subject to the objection of producing a heavy side tone in the receiver of the transmitting station.  By “side tone” is meant the noises which are produced in the receiver at a station by virtue of the action of the transmitter at that station.  Side tone is objectionable for several reasons:  first, it is sometimes annoying to the subscriber; second, and of more importance, the subscriber who is talking, hearing a very loud noise in his own receiver, unconsciously assumes that he is talking too loud and, therefore, lowers his voice, sometimes to such an extent that it will not properly reach the distant station.

[Illustration:  Fig. 131.  Bridging Battery with Impedance Coils]

Bridging Battery with Impedance Coils. The method of feeding current to the line from the common battery, shown in Fig. 130, is called the “split repeating-coil” method.  As distinguished from this is the impedance-coil method which is shown in Fig. 131.  In this the battery is bridged across the circuit of the combined lines in series with two impedance coils, 1 and 2, one on each side of the battery.  The steady currents from the battery find ready path through these impedance coils which are of comparatively low ohmic resistance, and the current divides and passes in multiple over the circuits of the two lines.  Voice currents, however, originating at either one of the stations, will not pass through the shunt across the line at the central office on account of the high impedance offered by these coils, and as a result they are compelled to pass on to the distant station and affect the receiver there, as desired.