tral office.

[Illustration: Fig. 202. Circuits of Roberts Line]

Since, however, all parties on the line are normally locked out of
talking circuits, some means must be provided whereby the operator may
place the signaling party in talking connection and leave all the
other instruments on the line in their normally locked-out position.
In fact, the operator must be able automatically to pick out the
station that signaled in, and operate the ringer to unlatch the
springs controlling the talking circuit of that station. Accordingly
the operator sends impulses on the line, from a grounded battery,
which are in the direction to operate the line relays and to continue
the line circuit to the station calling. When, after a sufficient
number of impulses, this current reaches that station it finds a path
to ground from the limb _L_. This path is made possible by the fact
that the subscriber's receiver is off its hook at that station. In
order to understand just how this ground connection is made, it must
be remembered that each of the ringer magnets is energized with each
selecting impulse, but in such a direction as not to ring the bells,
it being understood that all of the ringer mechanisms are normally
latched. When the selecting impulse for Station C arrives, it passes
through the ringer and the selecting relay coils at that station and
starts to operate the remainder of the ringers sufficiently to cause
the spring _12_ to engage the spring _13_. This establishes the ground
connection from the limb _L_ of the line, the circuit being traced
through limb _L_ through the upper contact of the switch, thence
through springs _12_ and _13_ to ground, and this, before the line
relay has time to latch, operates the quick-acting relay at the
central office, which acts to cut off further impulses, and thus
automatically stops at the calling station. Ringing current in the
opposite direction is then sent to line; this unlatches the ringer
springs and places the calling subscriber in talking circuit. When the
operator has communicated with the calling subscriber, and found, for
example, that another party on another similar line is desired, she
turns the dial pointer on the selector to the number corresponding to
the called-for party's number on that line, and presses the signal
key. Pressing this key causes impulses to "run down the line,"
selecting the proper party and ringing his bell in the manner already
described. The connectio