on
the "jack per station" basis. In "jack per station" working, the
selection of the station on a party line is determined by the jack on
which the plug is put, rather than by a ringing key, and hence the
keyless trunk may be employed.

[Illustration: Fig. 376. Keyless Trunk]

A keyless trunk as used in New York is shown in Fig. 376. This has no
manually operated keys whatever, and the relay _17_, when it is
operated, establishes connection between the ringing generator and the
conductors of the trunk plug. The relays _3_, _13_, and _12_ operate in
a manner identical with those bearing corresponding numbers in Fig.
371. As soon as the trunk operator plugs into the multiple jack of the
called subscriber, the relay _16_ will operate for the same reason that
the relay _9_ operated in connection with Fig. 371. The trunk disconnect
lamp will receive current, but if the operator has already established
connection with the other end of the trunk, this lamp will not be
lighted because shunted by the relay _17_, due to the pulling up of the
armature of the relay _13_. The relay _15_ plays no part in the
operation so far described, because of the fact that its winding is
short-circuited by its own contacts and those of relay _12_, when the
latter is not energized. As a result of the operation of the relay _17_,
ringing current is sent to line, the supply circuit including the coil
of the relay _12_. As soon as the subscriber responds to this ringing
current, the armature of the relay _12_ is pulled up, thus breaking the
shunt about the relay _15_, which, therefore, starts to operate in
series with the relay _17_, but as its armatures assume their attracted
position, the relay _17_ is cut out of the circuit, the coil of the
relay _15_ being substituted for that of the relay _17_ in the shunt
path around the lamp _4_. The relay _17_ falls back and cuts off the
ringing current. The relay _15_ now occupies the place with respect to
the shunt around the lamp _4_ that the relay _17_ formerly did, the
continuity of this shunt being determined by the energization of the
relay _13_. When the _A_-operator at the distant exchange withdraws the
calling plug from the trunk jack, this relay _13_ becomes de-energized,
breaking the shunt about the lamp _4_ and permitting the display of that
lamp as a signal to the operator to take down the connection. It may be
asked why the falling back of relay _15_ will not again energize relay
_17_ and th