ling subscriber, not knowing this, presses his ringing
button. This grounds the vertical side of the line at his station and
operates the vertical relay at the connector. This steps the shaft of
the connector up one step and causes the closure of the contacts _19_
and _20_ at the top of the connector shaft. This establishes a
connection to a circuit carrying periodically interrupted battery
current on which an inductive hum is placed. This circuit may be traced
from this source through the springs _20_ and _19_ to the first wiper of
the side switch, thence through the normally closed contacts of the
ringing relay to the rotary side of the line, and the varying potential
to which this path is subjected produces an inductive flow back to the
calling subscriber's telephone, and gives him the necessary signal which
consists of a hum or buzzing noise with which all users of automatic
systems soon become familiar.

Release on Busy Connection:--The connector, since its last release, has
been stepped up one notch and must again be released. When the
subscriber hangs up his receiver after receiving the busy signal, he
grounds both sides of his line momentarily by the action of the springs
_21_, _22_, and _23_ of Fig. 384. This operates the rotary and the
vertical relays on the connector simultaneously and brings together for
the first time the springs _21_ and _22_ of Fig. 396. This establishes a
connection from the battery through the springs _16_ and _17_ on the
calling battery supply relay, thence through the release magnet of the
connector, thence through the springs _22_ and _21_ of the vertical and
the rotary relay, thence through the release trunk back to the second
selector. From here the circuit passes through the private wiper of that
selector and the back release relay to ground through the third side
switch wiper which is in the third position. Considering this circuit in
respect to its action on the connector it is obvious that it energizes
the release magnet on the connector which restores the connector to
normal as before. At the second selector this circuit passed through the
back release relay, which closed a circuit through the release magnet
and through the back release relay contacts, thence back over the second
selector release trunk to the back release relay of the first selector,
and through the third wiper of the side switch on that selector to
ground, since that side switch also is in its third position.