a battery being included between the
coils in this circuit. The coils of the relays _3_ and _4_ are similarly
connected across the calling cord. A peculiar feature of the Kellogg
system is that two batteries are used in connection with the cord
circuit, one of them being common to all answering cords and the other
to all calling cords. The operation of the system would, however, be
exactly the same if a single battery were substituted for the two.

_Supervisory Signals._ Considering the relays associated with the
answering cord, it is obvious that these two relays _1_ and _2_ together
control the circuit of the supervisory lamp _5_, the circuit of this
lamp being closed only when the relay _1_ is de-energized and the relay
_2_ is energized. We will find in discussing the operation of these that
the relay _2_ is wholly under the control of the operator, and that the
relay _1_, after its plug has been connected with a line, is wholly
under the control of the subscriber on that line. It is through the
windings of these two relays that current is fed to the line of the
subscriber connected with the corresponding cord.

When a plug--the answering plug, for instance--is inserted into a jack,
current at once flows from the positive pole of the left-hand battery
through the winding of the relay _2_ to the sleeve of the plug, thence
to the sleeve of the jack and through the cut-off relay to ground. This
at once energizes the supervisory relay _2_ and the cut-off relay
associated with the line. The cut-off relay acts, as stated, to continue
the tip and sleeve wires associated with the jacks to the line leading
to the subscriber, and also to cut off the line relay. The supervisory
relay _2_ acts at the same time to attract its armature and thus
complete its part in closing the circuit of the supervisory lamp.
Whether or not the lamp will be lighted at this time depends on whether
the relay _1_ is energized or not, and this, it will be seen, depends on
whether the subscriber's receiver is off or on its hook. If off its
hook, current will flow through the metallic circuit of the line for
energizing the subscriber's transmitter, and as whatever current goes to
the subscriber's line must flow through the relay _1_, that relay will
be energized and prevent the lighting of the supervisory lamp _5_. If,
on the other hand, the subscriber's receiver is on its hook, no current
will flow through the line, the supervisory relay will not be e