tential of all the test rings so as to guard
the line against intrusion by other callers. The supervisory lamp _5_
remains unlighted because, although the relay _2_ is operated, the relay
_1_ is also operated, due to the calling subscriber's receiver being off
its hook. The operator throws her listening key, communicates with the
subscriber, and, learning that the right-hand station is wanted,
proceeds to test that line. If the line is idle, she will get no click,
because the tip of her calling plug and the tested ring will be at the
same ground potential. She then plugs in and presses the proper
ringing-key button to send out the proper frequency to ring the
particular subscriber on the line--if there be more than one--the
current from the battery through the coil _11_ and spring _17_ serving
during this operation to hold up the cut-off relay.

As soon as the operator plugs in with the calling plug, the supervisory
lamp _6_ lights, assuming that the called subscriber had not already
removed his receiver from its hook, due to the fact that the relay _4_
is energized and the relay _3_ is not. As soon as the called subscriber
responds, the relay _3_ becomes energized and the supervisory lamp goes
out. If the line called for had been busy by virtue of being plugged at
another section, the tip of the operator's plug in testing would have
found the test ring raised to a potential above the ground, and, as a
consequence, current would have flowed from the tip of this plug through
the back contact of the right-hand lever of relay _4_, thence through
the ringing key springs and the auxiliary listening-key springs to
ground through the retardation coil _14_. This would have produced a
click by causing a momentary flow of current through the tertiary
winding _16_ of the operator's set.

_Wiring of Line Circuit._ The more complete wiring diagram of a single
subscriber's line, Fig. 353, shows the placing in the circuits of the
terminals and jumper wires of the main distributing frame and of the
intermediate distributing frame, and also shows how the pilot lamps and
night-alarm circuits are associated with a group of lines. The main
distributing frame occupies the same relative position in this line
circuit as in the Western Electric, being located in the main line
circuit outside of all the switchboard apparatus. The intermediate
distributing frame occupies a different relative position from that in
the Western Electric line. It w