that
the insertion of a plug in the other end of the trunk will, by breaking
contact between springs _1_ and _2_, put out both the lamps. One plug
inserted will, therefore, light both lamps; two plugs inserted or two
plugs withdrawn will extinguish both lamps.

[Illustration: Fig. 331. Jack-Ended Transfer Circuit]

If an operator located at one end of this trunk answers a call and finds
that the called-for subscriber's line terminates within reach of the
operator near the other end of this trunk, she will insert a calling
plug, corresponding to the answering plug used in answering a call, into
the jack of this trunk and thus light the lamp at both its ends. The
operator at the other end upon seeing this transfer lamp illuminated
inserts one of her answering plugs into the jack, and by means of her
listening key ascertains the number of the subscriber desired, and
immediately inserts her calling plug into the jack of the subscriber
wanted and rings him in the usual manner. The act of this second
operator in inserting her answering plug into the jack extinguishes the
lamp at her own end and also at the end where the call originated, thus
notifying the answering operator that the call has been attended to. As
long as the lamps remain lighted, the operators know that there is an
unattended connection on that transfer line. Such a transfer line is
called a _two-way_ line or a _single-track_ line, because traffic over
it may be in either direction. In Fig. 331 is shown a trunk that
operates in a similar way except that the two lamps, instead of being
arranged in multiple, are arranged in series.

[Illustration: Fig. 332. Jack- and Plug-Ended Transfer Circuit]

_Plug-Ended Trunk._ In Fig. 332 is shown a plug-ended trunk, this
particular arrangement of circuits being employed by the Monarch Company
in its transfer boards. This is essentially a one-way trunk, and traffic
over it can pass only in the direction of the arrow. Traffic in the
opposite direction between any two operators is handled by another trunk
or group of trunks similar to this but "pointed" in the other direction.
For this reason such a system is referred to as a _double-track_ system.
The operation of signals is the same in this case as in Fig. 330, except
that the switching device at the left-hand end of the trunk instead of
being associated with the jack is associated with the plug seat, which
is a switch closely associated with the seat of a plug so as to