the springs _4_ and _5_ of the bridged
relay _6_ and thence to the vertical side of the trunk and to the
distant office, where it passes into a second selector and through its
vertical relay to battery. Thus the vertical impulses are passed on over
the two-wire trunk to the second selector at the distant office. It
becomes necessary, however, to prevent these impulses from passing back
through the winding of the bridge relay _6_ and this is done by means of
the sluggish relay _7_. This relay receives local battery impulses in
unison with those sent over the trunk by the vertical relay, these being
supplied from the battery at the local office through the contacts _8_
and _9_ of the calling battery supply relay, which works in unison with
the vertical relay. These rapidly recurring impulses are too fast for
the sluggish relay _7_ to follow. And this relay merely pulls up its
armature and cuts off both sides of the trunk leading back to the first
selector. The rotary impulses are repeated to the rotary side of the
two-wire trunk in a similar way.

Considering now the operation of the trunk repeater in the reverse
direction, the action of the bridging relay _6_ is of vital importance.
Normally both sides of trunk line are connected to the live side of the
battery and, therefore, there is no difference of potential between them
and no tendency to operate the bridged relay. When the connection has
been fully established to the subscriber at the distant office, and that
subscriber has responded, the action of his battery supply relay will,
as before stated, change the connection of the rotary side of the line
from battery to ground, and thus bridge the battery at the distant
exchange across the trunk. This action will pull up the bridged relay
_6_ at the trunk repeater and will perform exactly the same function
with respect to the connection of the battery with the calling
subscriber's line. In other words, it will change the connection of the
rotary side of the calling line from battery to ground, thus
establishing the necessary difference in potential to give the calling
subscriber the necessary current for transmission purposes. The
disconnect feature is about the same as already described. When the
calling subscriber hangs up his receiver both the vertical and rotary
relays of the trunk repeater operate, which places the ground on both
sides of the two-wire trunk to the distant office, which is the
condition for releasing