d _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 transmit