operation of any one of the other springs. As a result the proper
ringing circuit is established, it being understood that the upper
contacts of the springs _7_, _8_, _9_, and _10_ lead to the terminals of
their respective ringing generators, the other terminals of which are
grounded. The circuit is, therefore, from the generator, through the
ringing key, out through the tip side of the line, back over the sleeve
side of the line, and to ground through the spring _17_, resistance
_11_, and the battery, which is one of the cord-circuit batteries. The
object of this coil _11_ and the battery connection through it to the
ringing-key spring is to prevent the falling back of the cut-off relay
when the ringing key is operated. This will be clear when it is
remembered that the cut-off relay is energized by battery current fed
over the sleeve strand of the cord, and obviously, since it is necessary
when the ringing key is operated to cut off the supply wire back of the
key, this would de-energize the cut-off relay when the ringing key was
depressed, and the falling back of the cut-off relay contacts would make
it impossible to ring because the sleeve side of the line would be cut
off. The battery supply through the resistance _11_ is, therefore,
substituted on the sleeve strand of the cord for the battery supply
through the normal connection.

_Busy Test._ The busy test depends on all of the test rings being at
zero potential on an idle line and at a higher potential on a busy line.
Obviously, when the line is not switched, the test rings are at zero
potential on account of a ground through the cut-off relay. When,
however, a plug is inserted in either the answering or multiple jacks,
the test rings will all be raised in potential due to being connected
with the live side of the battery through the sleeve strand of the cord.
Conditions on the line external to the central office cannot make an
idle line test busy because, owing to the presence of the cut-off relay,
the sleeve contacts of all the jacks are disconnected from the line when
it is idle. The test circuit from the tip of the calling plug to ground
at the operator's set passes through the tip strand of the cord, thence
through a pair of normally closed extra contacts on the supervisory
relay _4_, thence in series through all the ringing key springs _10_,
_9_, _8_, and _7_, thence through an extra pair of springs _12_ and _13_
on the listening key--closed only when the l