pt her finger on the key. The other normally
open point in the circuit of the holding magnet _8_ is at the lower pair
of contacts of the test and holding relay _9_. This relay is operated
whenever the trunk plug is inserted in the jack of a called line,
regardless of the position of the subscriber's equipment on that line.
The circuit may be traced from the live side of the battery through the
trunk disconnect lamp _4_, coil _9_, sleeve strand of cord, and to
ground through the cut-off relay of the line. The insertion of the trunk
plug into the jack thus leaves the completion of the holding-magnet
circuit dependent only upon the auxiliary contact on the ringing key,
and, therefore, as soon as the operator presses either one of these
keys, the clutch magnet is energized and the key is held down, so that
ringing current continues to flow at regular intervals to the called
subscriber's station.

The ringing current issues from the generator _10_, but the supply
circuit from it is periodically interrupted by the commutator _11_
geared to the ringing-machine shaft. This periodically interrupted
ringing current passes to the ringing-key contacts through the coil of
the ringing cut-off relay _12_, and thence to the subscriber's line. The
ringing current is, however, insufficient to cause the operation of this
relay _12_ as long as the high resistance and impedance of the
subscriber's bell and condenser are in the circuit. It is, however,
sufficiently sensitive to be operated by this ringing current when the
subscriber responds and thus substitutes the comparatively low
resistance and impedance path of his talking apparatus for the previous
path through his bell. The pulling up of the ringing cut-off relay _12_
breaks a third normally closed contact in the circuit of the holding
coil _8_, de-energizing that coil and releasing the ringing key, thus
cutting off ringing current. There is a third brush on the commutator
_11_ connected with the live side of the central battery, and this is
merely for the purpose of assuring the energizing of the ringing cut-off
relay _12_, should the subscriber respond during the interval while the
commutator _11_ held the ringing current cut off. The relay _12_ may
thus be energized either from the battery, if the subscriber responds
during a period of silence of his ringer, or from the generator _10_, if
the subscriber responds during a period while his bell is sounding; in
either case the ringing