266,798. Aitken, I. M. E., Gordon, D. A., and Metropolitan-Vickers Electrical Co.. Ltd. Nov. 26, 1925. Selective signalling apparatus.-In a system for signalling from any one of a plurality of remote points to a central station of the kind in which graded resistance elements located at the remote points are connected by a pair of line wires in the fourth arm of a Wheatstone bridge, a plurality of automatic stepping switches adapted to vary the third arm of the bridge at the central office is provided in order that a number of co-existent signals at the remote points may be registered. Fig. 1 shows a signalling system in which the positions of switches 31, 32, 35 &c. located at different remote points are indicated by lamps 711. Each switch may be operated manually or in response to a predetermined condition such as shutting down or starting up of apparatus. The switches are connected in series with resistances 41, 42, 45 having characteristic values, across line wires 503, 509 which form the fourth arm of a Wheatstone bridge comprising the ratio arms 6, 7 and a. group of resistances 511 - - 517 located at the central office and corresponding in value to the resistances at the remote points. When a switch such as 35 is closed, the bridge becomes unbalanced and a circuit is completed at the contact 802 of a polarized relay 508 for the stepping magnet 170 of a switch 100 which is moved step-by-step over the bank of contacts 111 - - 117 to select the appropriate resistance in the group 511 - - 517 and rebalance the bridge. A wiper 160 is simultaneously moved to operate a lamp in the group 711 individual to the operated switch 35. If a second switch at a remote staion is also operated, the relay 508 is again energized and the switch 100 is stepped forward until when the wiper 140 engages the contact 119 the stepping switch 200 is operated one step to select the first resistance, 511 and the switch 100 is restored to its normal position. As the bridge is still unbalanced, the switch 100 is reoperated in search of a second resistance. This operation, i.e. a movement of the switch 200 through one step for each revolution of the switch 100 is repeated until the two appropriate resistances in the group 511 - - 517 are connected in the bridge circuit and the stepping operation is terminated. The switch 200 has a wiper 260 co-operating with contacts 230 - - 237 connected to the lamps 711. When a switch is opened, the relav 508 closes, the circuit of the coils 179 279, which release the stepping switches 100, 200. After release, the switches are operated into fresh positions corresponding to the operated switches at the remote stations. The indications of the lamps 711 may be checked by a key 809 which causes the release and re-operation of the stepping switches. A relay 804 is energized to prevent further operation of the switches when the switch 200 engages its last contact 219. This condition is indicated by a device 719 energized over wiper 260. In the modification shown in Fig. 2, unbalancing of the bridge by the operation of one or more of the remote switches 31 - - 35 causes the relay 508 to energize the release coil 819 of a master stepping switch 810 and a slow-acting relay 830 which energizes the magnet 841 to release a drop switch 842 and operate an alarm device 849. Simultaneously, the stepping magnet 811 is energized to operate the arms 817, 818 over their respective banks of contacts. The contacts 2100, 2200 &c. associated with the arm 818 are connected respectively to the zero contacts of stepping switches 100, 200 &c. The arms 817, 818 are stepped forward until the arm 818 engages the contact associated with the first available switch 100, 200 &c. which occupies a normal position, when the magnet 811 is short-circuited and the stepping is stopped. After a short interval of time, the relay 850, which is previously energized by the relay 831, closes the circuit of the stepping magnet of the selected switch 100, 200 &c. over the wiper 817 of the master switch. The selected switch is then operated step-by-step until the appropriate resistance in the group 511 - - 517 balances the bridge and de-energizes the relay 508. The switch also operates an arm such as 160, 260 &c. to select the appropriate signal device in the group 711 - - 717. When one of the switches at the remote point is opened, the relay 508 engages the contact 803. A relay 870 is energized to operate the relay 841 and give an alarm and simultaneously energize the stepping magnet 821 of the switch 820. The wiper 828 of the switch 820 is advanced to select a resistance in the group 611, 612 &c. and again balance the bridge and stop the switch. Upon de-energization of the relay 870, a circuit is established over the release coil 829, wiper 827, wiper and release coil -of the stepping switch which at the moment selects a resistance corresponding to that of the remote switch. The switch 820 and the stepping switch are thus returned to zero, and the indication is cancelled. In the event of a short-circuit across the line wires 503, 509, a relay 861 is energized to release the drop switch 862, operate an alarm 869 and open the line circuit. The line wires may be used for telephonic signalling, a certain value of resistance being connected to the line wires to operate the switches at the central station to a position indicating that a telephone call has originated. The Provisional Specification states that impedance elements may be used and describes an arrangement wherein the Wheatstone bridge is re-balanced at the central station by a manually operated switch.