As will be seen, the current flowing from the positive pole of the battery will divide and flow through the windings 2 and 4; thence over the upper limb of each line, through the transmitter at each station, and back over the lower limbs of the line, through the windings 1 and 3, where the two paths reunite and pass to the negative pole of the battery.  It is evident that when neither transmitter is being used the current flowing through both lines will be a steady current and that, therefore, neither line will have an inductive effect on the other.  When, however, the transmitter at Station A is used the variations in the resistance caused by it will cause undulations in the current.  These undulations, passing through the windings 1 and 2 of the repeating coil, will cause, by electromagnetic induction, alternating currents to flow in the windings 3 and 4, and these alternating currents will be superimposed on the steady currents flowing in that line and will affect the receiver at Station B, as will be pointed out.  The reverse conditions exist when Station B is talking.

Bell Substation Arrangement. The substation circuits at the stations in Fig. 130 are illustrative of one of the commonly employed methods of preventing the steady current from the battery from flowing through the receiver coil.  This particular arrangement is that employed by the common-battery instruments of the various Bell companies.  Considering the action at Station B, it is evident that the steady current will pass through the transmitter and through the secondary winding of the induction coil, and that as long as this current is steady no current will flow through the telephone receiver.  The receiver, transmitter, and primary winding of the induction coil are, however, included in a local circuit with the condenser.  The presence of the condenser precludes the possibility of direct current flowing in this path.  Considering Station A as a receiving station, it is evident that the voice currents coming to the station over the line will pass through the secondary winding and will induce alternating currents in the primary winding which will circulate through the local circuit containing the receiver and the condenser, and thus actuate the receiver.  The considerations are not so simple when the station is being treated as a transmitting station.  Under this condition the steady current passes through the transmitter in an obvious manner.  It is clear that if the local circuit containing the receiver did not exist, the circuit would be operative as a transmitting circuit because the transmitter would produce fluctuations in the steady current flowing in the line and thus be able to affect the distant station.  The transmitter, therefore, has a direct action on the currents flowing in the line by the variation in resistance which it produces in the line circuit.  There is, however, a subsidiary action in this circuit.  Obviously,