5. Roberts Latching Relay]

The diagram of Fig. 196 shows, in simplified form, a four-station
line, the circuits being given more in detail than in the diagrams of
Chapter XVI.

It will be noticed that the ringer and the relay coil _6_ at the
first station are bridged across the sides of the line leading to the
central office. In like manner the bell and the relay magnets are
bridged across the two limbs of the line leading into each succeeding
station, but this bridge at each of the stations beyond Station A is
ineffective because the line extension _R__{x} is open at the next
station nearest the central office.

[Illustration: Fig. 196. Simplified Circuits of Roberts System]

In order to ring Station A it is only necessary to send out ringing
current from the central office. This current is in such direction as
not to cause the operation of the relay, although it passes through
the coil _6_. If, on the other hand, it is desired to ring Station B,
a preliminary impulse would be sent over the metallic circuit from the
central office, which impulse would be of such direction as to operate
the relay at Station A, but not to operate the bell at that station.
The operation of the relay at Station A causes the spring _2_ of this
relay to engage the spring _3_, thus extending the line on to the
second station. After the spring _2_ at Station A has been forced into
contact with the spring _3_, it is caught by the latch of the spring
_5_ and held mechanically. When the impulse from the central office
ceases, the spring _1_ resumes its normal position, thus breaking the
bridge circuit through the bell at that station. It is apparent now
that the action of coil _6_ at Station A has made the relay powerless
to perform any further action, and at the same time the line has been
extended on to the second station. A second similar impulse from the
central office will cause the relay at Station B to extend the line on
to Station C, and at the same time break the circuit through the
operating coil and the bell at Station B. In this way any station may
be picked out by sending the proper number of impulses to operate the
line relays of all the stations between the station desired and the
central office, and having picked out a station it is only necessary
to send out ringing current, which current is in such direction as to
ring the bell but not to operate the relay magnet at that station.

In Fig. 197, a four-station line, such as