The windings 7 are the true unlocking windings, while the windings 7^{1} have no other function than to neutralize the inductive effects of these unlocking windings necessarily placed in series in the talking circuit.  All of these windings are of low ohmic resistance, a construction which, as has previously been noted, brings about the desired effect without introducing any self-induction in the line, and without producing any appreciable effect upon the transmission.  A study of Fig. 198 will make clear the connections of these unlocking and balancing windings at each station.

The statement of operation so far given discloses the general method of building up the line in sections in order to choose any party and of again breaking it up into sections when the conversation is finished.  It has been stated that the same operation which selects the party wanted also serves to give that party the use of the line and to lock the others off.  That this is true will be understood when it is stated that the ringer is of such construction that when operated to ring the subscriber wanted, it also operates to unlatch a set of springs similar to those shown in Fig. 194, this unlatching causing the proper connection of the subscriber’s talking circuit across the limbs of the line, and also closing the local circuit through his transmitter.  The very first motion of the bell armature performs this unlatching operation after which the bell behaves exactly as an ordinary polarized biased ringer.

[Illustration:  Fig. 199.  Broken-Back Ringer]

The construction of this ringer is interesting and is shown in its two possible positions in Fig. 199.  The group of springs carried on its frame is entirely independent of the movement of the armature during the ringing operation.  With reversed currents, however, the armature is moved in the opposite direction from that necessary to ring the bells, and this causes the latching of the springs into their normal position.  In order that this device may perform the double function of ringer and relay the tapper rod of the bell is hinged on the armature so as to partake of the movements of the armature in one direction only.  This has been called by the inventor and engineers of the Roberts system a broken-back ringer, a name suggestive of the movable relation between the armature and the tapper rod.  The construction of the ringer is of the same nature as that of the standard polarized ringer universally employed, but a hinge action between the armature and the tapper rod, of such nature as to make the tapper partake positively of the movements of the armature in one direction, but to remain perfectly quiescent when the armature moves in the other direction, is provided.

[Illustration:  Fig. 200.  Details of Ringer Connection]

How this broken-back ringer controls the talking and the locking-out conditions may best be understood in connection with Fig. 200.  The ringer springs are normally latched at all stations.  Under these conditions the receiver is short-circuited by the engagement of springs 10 and 11, the receiver circuit is open between springs 10 and 12, and the local-battery circuit is open between springs 9 and 12.  The subscribers whose ringers are latched are, therefore, locked out in more ways than one.