us cause a false ring on the called subscriber. This will not
occur because both the relays _15_ and _17_ depend for their
energization on the closure of the contacts of the relay _13_, and when
this falls back the relay _17_ cannot again be energized even though the
relay _15_ assumes its normal position.

=Kellogg Trunk Circuits.= The provision for proper working of trunk
circuits in connection with the two-wire multiple switchboards is not an
altogether easy matter, owing particularly to the smaller number of
wires available in the plug circuits. It has been worked out in a highly
ingenious way, however, by the Kellogg Company, and a diagram of their
incoming trunk circuit, together with the associated circuits involved
in an inter-office connection, is shown in Fig. 377.

[Illustration: Fig. 377. Inter-Office Connection--Kellogg System]

This figure illustrates a connection from a regular two-wire multiple
subscriber's line in one office, through an _A_-operator's cord circuit
there, to the outgoing trunk jacks at that office, thence through the
incoming trunk circuit at the other office to the regular two-wire
multiple subscriber's line at that second office. The portion of this
diagram to be particularly considered is that of the _B_-operator's cord
circuit. The trunk circuit terminates in the multipled outgoing trunk
jacks at the first office, the trunk extending between offices
consisting, of course, of but two wires. We will first consider the
control of the calling supervisory lamp in the _A_-operator's cord
circuit, it being remembered that this control must be from the called
subscriber's station. It will be noticed that the left-hand armature of
the relay _1_ serves normally to bridge the winding of relay _2_ across
the cord circuit around the condenser _3_. When, however, the relay _1_
pulls up, the coil of relay _4_ is substituted in this bridge connection
across the trunk. The relay _2_ has a very high resistance
winding--about 15,000 ohms--and this resistance is so great that the tip
supervisory relay of the _A_-operator's cord will not pull up through
it. As a result, when this relay is bridged across the trunk circuit,
the tip relay on the calling side of the _A_-operator's cord circuit is
de-energized, just as if the trunk circuit were open, and this results
in the lighting of the _A_-operator's calling supervisory lamp. The
winding of the relay _4_, however, is of low resistance--about 50
ohms--an