nal contact on
the listening key so as to restore the clearing-out drop whenever the
listening key was operated.

_Operation._ An understanding of the operation of this system is easy.
The turning of the subscriber's generator, when the line was in its
normal condition, caused the display of the line signal. The insertion
of the answering plug, in response to this call, did three things: (1)
It extended the line circuit to the tip and sleeve strand of the cord
circuit. (2) It energized the restoring coil _6_ of the drop by
establishing the circuit from the contact spring _3_ through the plug
contact _8_ to the other contact spring _4_, thus completing the circuit
between the two normally open auxiliary wires. (3) The connecting of the
springs _3_ and _4_ established a connection from ground to the test
thimbles of all the jacks on a line, the spring _4_ being always
grounded and the spring _3_ being always connected to the test thimble
_5_.

It is to be noted that on idle lines the test rings are always at the
same potential as the ungrounded pole of the battery _7_, being
connected thereto through the winding _6_ of the restoring coil. On all
busy lines, however, the test rings are dead grounded through the
contact _8_ of the plug that is connected with the line.

The tip of the testing plug at the time of making a test will also be at
the same potential as that of the ungrounded pole of the battery _7_,
since this pole of the battery _7_ is always connected to the center
portion of the operator's receiver winding, and when the listening key
is thrown the tip of the calling plug is connected therewith and is at
the same potential. When, therefore, the operator touches the tip of
the calling plug to the test thimble of an idle line, she will get no
click, since the tip of the plug and the test thimble will be at the
same potential. If, however, the line has already been switched at
another section of the board, there will be a difference of potential,
because the test thimble will be grounded, and the circuit, through
which the current which causes the click flows, may be traced from the
ungrounded pole of the battery _7_ to the center portion of the
operator's receiver, thence through one-half of the winding to the tip
of the calling plug, thence to the test thimble of the jack under test,
thence to the spring _3_ of the jack on another section at which the
connection exists, through the contact _8_ on the plug of t