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.

When the bell is rung, the first stroke it makes unlatches the springs,
which assume the position shown in the right-hand cut of Fig. 199, and
this, it will be seen from Fig. 200, establishes proper conditions for
enabling the subscriber to transmit and to receive speech.

The hook switch breaks both transmitter and receiver circuits when down
and in raising it establishes a momentary circuit between the ground
and the limb _L_ of the line, both upper and lower hook contacts
engaging the hook lever simultaneously during the rising of the hook.

The mechanism at the central office by which selection of the proper
station is made in a rapid manner is shown in Fig. 201. It has already
been stated that the selection of the proper subscriber is brought
about by the sending of a predetermined number of impulses from the
central office, the