is shown in Fig. 196, is
illustrated, but the condition shown in this is that existing when two
preliminary impulses have been sent over the line, which caused the
line relays at Station A and Station B to be operated. The bell at
Station C is, therefore, the only one susceptible to ringing current
from the central office.

[Illustration: Fig. 197. Simplified Circuits of Roberts System]

Since only one bell and one relay are in circuit at any one time, it
is obvious that all of the current that passes over the line is
effective in operating a single bell or relay only. There is no
splitting up of the current among a large number of bells as in the
bridging system of operating step-by-step devices, which method
sometimes so greatly reduces the effective current for each bell that
it is with great difficulty made to respond. All the energy available
is applied directly to the piece of apparatus at the time it is being
operated. This has a tendency toward greater surety of action, and the
adjustment of the various pieces of apparatus may be made with less
delicacy than is required where many pieces of apparatus, each having
considerable work to do, must necessarily be operated in multiple.

The method of unlatching the relays has been briefly referred to.
After a connection has been established with a station in the manner
already described, the operator may clear the line when it is proper
to do so by sending impulses of such a nature as to cause the line
relays of the stations beyond the one chosen to operate, thus
continuing the circuit to the end of the line. The operation of the
line relay at the last station brings into circuit the coil _8_, Figs.
196 and 197, of a grounding device. This is similar to the line relay,
but it holds its operating spring in a normally latched position so as
to maintain the two limbs of the line disconnected from the ground.
The next impulse following over the metallic circuit passes through
the coil _8_ and causes the operation of this grounding device which,
by becoming unlatched, grounds the limb _L_ of the line through the
coil _8_. This temporary ground at the end of the line makes it
possible to send an unlocking or restoring current from the central
office over the limb _L_, which current passes through all of the
unlocking coils _7_, shown in Figs. 194, 196, and 197, thus causing
the simultaneous unlocking of all of the line relays and the
restoration of the line to its normal