[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 connection between the two parties is then established, and no one else can in any possible way, except by permission of the operator, obtain access to the line.

It is obvious that some means must be provided for restoring the selecting relays to normal after a conversation is finished.  By referring to Fig. 194 it will be seen that the upper end of the latch spring 5 is bent over in such a manner that when the armature is attracted by current flowing through the coil 7, the knob on the left-hand end of the armature on rising engages with the bent cam surface and forces back the latch, permitting spring 2 to return to its normal position.