hod of accomplishing
selective signaling and locking-out on telephone party lines is due to
Homer Roberts and his associates.

[Illustration: Fig. 194. Roberts Latching Relay]

To understand just how the principles illustrated in Figs. 186 and 187
are put into effect, it will be necessary to understand the latching
relay shown diagrammatically in its two possible positions in Fig. 194,
and in perspective in Fig. 195. Referring to Fig. 194, the left-hand
cut of which shows the line relay in its normal position, it is seen
that the framework of the device resembles that of an ordinary
polarized ringer. Under the influence of current in one direction
flowing through the left-hand coil, the armature of this device
depresses the hard rubber stud _4_, and the springs _1_, _2_, and _3_
are forced downwardly until the spring _2_ has passed under the latch
carried on the spring _5_. When the operating current through the coil
_6_ ceases, the pressure of the armature on the spring _1_ is relieved,
allowing this spring to resume its normal position and spring _3_ to
engage with spring _2_. The spring _2_ cannot rise, since it is held by
the latch _5_, and the condition shown in the right-hand cut of Fig.
194 exists. It will be seen that the spring _2_ has in this operation
carried out just the same function as the switch lever performed as
described in connection with Figs. 186 and 187. An analysis of this
action will show that the normal contact between the springs _1_ and
_2_, which contact controls the circuit through the relay coil and the
bell, is not broken until the coil _6_ is de-energized, which means
that the magnet is effective until it has accomplished its work. It is
impossible, therefore, for this relay to cut itself out of circuit
before it has caused the spring _2_ to engage under the latch _5_. If
current of the proper direction were sent through the coil _7_ of the
relay, the opposite end of the armature would be pulled down and the
hard rubber stud at the left-hand end of the armature would bear
against the bent portion of the spring _5_ in such manner as to cause
the latch of this spring to release the spring _2_ and thus allow the
relay to assume its normal, or unlatched, position.

A good idea of the mechanical construction of this relay may be
obtained from Fig. 195. The entire selecting function of the Roberts
system is performed by this simple piece of apparatus at each station.

[Illustration: Fig. 19