erator's cord. This circuit may be traced as follows: from battery
through the disconnect lamp, through the armatures and contacts of
relays _2_ and _4_, thence through the front contact and armature of
relay _5_ to ground, thus illuminating the disconnect lamp. The ringing
lamp will not be re-illuminated at this time, due to the fact that it
has been previously locked out by relay _6_. The operator then removes
the plug from the jack of the line called, and the apparatus in the
trunk circuit is restored to normal condition.

In the circuit shown only keys are provided for ringing two parties.
This circuit, however, is not confined to the use of two-party lines,
but may be extended to four parties by simply duplicating the ringing
keys and by connecting them with the proper current for selectively
ringing the other stations.

The method of determining as to whether the called line is free or busy
is similar to that previously described for the _A_-operator's cord
circuit when making a local connection, and differs only in the fact
that in the case of the trunk cord the test circuit is controlled
through the contacts of a relay, whereas in the case of the
_A_-operator's cord, the test circuit was controlled through the
contacts of the listening key. The function of the resistance _10_ and
the battery connected thereto is the same as has been previously
described.

The general make-up of trunking switchboard sections is not greatly
different from that of the ordinary switchboard sections where no
trunking is involved. In small exchanges where ring-down trunks are
employed, the trunk line equipment is merely added to the regular jack
and drop equipment of the switchboard. In common-battery multiple
switchboards the _A_-boards differ in no respect from the standard
single office multiple boards, except that immediately above the
answering jacks and below the multiple there are arranged in suitable
numbers the jacks of the outgoing trunks.

Where the offices are comparatively small, the incoming trunk portions
of the _B_-boards are usually merely a continuance of the _A_-boards,
the subscriber's multiple being continuous with and differing in no
respect from that on the _A_-sections. Instead of the usual pairs of
_A_-operators' plugs, cords, and supervisory equipment, there are on the
key and plug shelves of these _B_-sections the incoming trunk plugs and
their associated equipment.

In large offices it is customary to