[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 condition, as shown in Fig. 196.

[Illustration:  Fig. 198.  Details of Latching Relay Connections]

As has been stated, the windings 7 on the line relays are the unlatching windings.  In Figs. 196 and 197, for the purpose of simplicity, these windings are not shown connected, but as a matter of fact each of them is included in series in the continuous limb L of the line.  This would introduce a highly objectionable feature from the standpoint of talking over the line were it not for the balancing coils 7^{1}, each wound on the same core as the corresponding winding 7, and each included in series in the limb R of the line, and in such direction as to be differential thereto with respect to currents passing in series over the two limbs of the line.