our
successive movements of the dial will select the numbers in that ten
thousand in just the some way that has been already described.

Certain difficulties arise, however, in multi-office working due to the
fact that the three-wire trunks between offices would in most cases be
objectionable. As long as the trunks extend between the various groups
of apparatus in the same office, it is cheaper to provide three wires
for each of them than it is to make any additional complication in the
apparatus. Where the trunking is done between offices, however, the
system may be so modified as to work over two wire inter-office trunks.

_The Trunk Repeater._ The purpose of the trunk repeater is to enable the
inter-office trunking to be done over two wires. It may be said that the
trunk repeater is a device placed in the outgoing trunk circuit at the
office in which a call originates, which will do over the two wires of
the trunk leading from it to the distant office just the same thing that
the subscriber's signal transmitter does over the two wires of the
subscriber's lines. It has certain other functions in regard to feeding
the battery for talking purposes back to the calling subscriber's line,
taking the place in this respect of the calling battery feed relay in
the connector in a single office exchange.

[Illustration: Fig. 397. Circuits of Trunk Repeater]

The circuits of a trunk repeater are shown in Fig. 397. In considering
it, it must be understood that the three wires entering the figure at
the left are the vertical, rotary, and release wires of a second
selector trunk leading from the first selector banks in the same office.
The two wires leading from the right of the figure are those extending
to the distant office, and terminate there in second selectors. The
vertical and the rotary sides of this trunk as shown at the left will
receive the impulses from the subscriber's station coming through the
line switch and the first selector, as usual. The vertical impulses will
pass through the winding of the vertical relay and through the winding
_1_ of the calling battery supply relay and thence to battery, the same
as on a connector. These impulses will work the armatures of both of
these relays in unison. The movements of the vertical relay armature in
response to these impulses will cause corresponding impulses to flow
over a circuit which may be traced from ground, through the springs _3_
and _2_ of the vertical relay,