aced from the
negative or ungrounded side of the battery to the center portion of the
cord circuit, thence through the supervisory lamp _1_, resistance _2_,
to the third conductor on the plug, test thimble on the jack, thence
through the winding of the cut-off relay to ground, which forms the
other terminal of the battery. The results accomplished by the closing
of this circuit are: first, the energizing of the cut-off relay to cut
off the signaling portion of the line; second, the flowing of current
through the lamp that is almost sufficient to illuminate it, but not
quite so because of the closure of the shunt about it, for the reason
that will be described; third, the raising of the potential of all the
contact thimbles on the jacks from zero to a potential different from
that of the ground and equal in amount to the fall of potential through
the winding of the cut-off relay. A condition is thus established at the
test rings such that some other operator at some other section in
testing the line will find it busy and will not connect with it.

[Illustration: Fig. 347. Western Electric No. 1 Board]

The reason why the lamp _1_, connected with the answering plug, was not
lighted was that the supervisory relay _3_, associated with the
answering plug, became energized when the operator plugged in, due to
the flow of current from the battery through the calling subscriber's
talking apparatus, this flow of current being permitted by the removal
of the calling subscriber's receiver from its hook. The energizing of
this relay magnet by causing the attraction of its armature, closed the
shunt about the lamp _1_, which shunt contains the 40-ohm resistance
_4_, and thus prevents the lamp from receiving enough current to
illuminate it. Obviously, as soon as the calling subscriber replaces his
receiver on its hook, the supervisory relay _3_ will be de-energized,
the shunt around the lamp will be broken, and the lamp will be
illuminated to indicate to the operator the fact that the subscriber
with whose line her calling plug is connected has replaced his receiver
on its hook.

_Testing--Called Line Idle._ Having now shown how the operator connects
with the calling subscriber's line and how that line automatically
becomes guarded as soon as it is connected with, so that no other
operator will connect with it, we will discuss how the operator tests
the called line and subsequently connects with that line, if it is found
proper to