Patent GB-598296-A (GB)
Title: Telecommunication systems
IPC: H04Q3/00
Filed: 1944-04-28
Abstract: 598,296. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. (International Standard Electric Corporation). April 28, 1944, No. 7982. [Class 40 (iv)] In a finder system employing cold-cathode tubes for detecting a calling potential which results directly from the closure of the calling loop, the a wire of each line is grounded through a high resistance 3 while the b wire, which is connected through a similar high resistance 4 to the c wire, is normally without potential. A low resistance leak from the b wire to ground or to the a wire thus simulates a calling condition and can be readily detected. In Fig. 2, the control anode 21 of the cold-cathode tube 20 is connected through a high resistance 7 to a potentiometer tapping 8 adjusted to -111 volts and to the wiper 5 of a constantly rotating switch to which the c wires are connected, so that when a calling condition is encountered, the negative potential on 21 is momentarily reduced sufficiently to ionize the tube 20. Relay 24 energizes in the main discharge path and connects up relay 25 which opens relay 24 to stop the discharge and disconnects the control anode 21 from wiper 5. Relay 26 now energizes in series with 24, operates the clutch magnet 51 of the allotted finder and connects the control anode 21 to the test wiper. When the calling line is found, the discharge occurs and relay 24 energizes as before, stops the finder and prepares a circuit for relay 50 which is completed by relay 27 when the first finder is picked up by a secondary finder. Relay 50 releases relays 24, 25, 26 and stops the discharge of the tube 20. In Fig. 3, the rotating switch acts also as an allotter and consists of a conductive ball 14 running between an outer ring of contacts 15 and an inner ring of contact arcs 16, 17, the contacts 15<1> opposite the arcs 17 being wired to the finders. A calling potential on an arc 16 ionizes the tube 20, the main discharge being maintained over a resistance 35, corresponding to relay 24, Fig. 2, and a second resistance 34, so that the negative potential on the cathode 22 and therefore on the arc 17 is reduced from 125 volts to, say, 75 volts. As the ball 14 continues, this potential ionizes the cold-cathode tube 40 of an idle finder and relay 52 energizes in the main discharge path in series with resistance 35, and the sudden drop of potential across this resistance de-ionizes the tube 20. Relay 52 locks, connects test relay 53 to the anode 43 of the tube 40, which is de-ionized during the change-over, and operates the clutch magnet 51 of the finder. The control anode 41 of the tube 40 is now connected to the test wiper over off-normal contact 54 so that ionization occurs and relay 53 pulls up when the calling line is reached ; this relay locks, stops the finder and releases relay 52, the tube 40 being de-ionized. Upon release, the finder goes to a normal position in which off-normal contacts 55 open. The finders serving a group of lines are given different normal positions and the method of allotment is such that the finder standing nearest to the calling line is first-choice. Fig. 4 shows an arrangement for 6 finders serving a group of 48 lines, the allotting switch having six pairs of finder contacts interposed between the line contacts, each finder being connected to two such contacts. For the sub-group of lines L1 ... L8, for example, the first-choice finder is ILF1 which stands adjacent to that sub-group, while the second-choice finder is ILF6 which has to move over only one subgroup to reach the subgroup in question. In another arrangement, four pairs of finders are employed, each pair of finders having the same normal position, and four sets of 8 finder contacts are interposed between the line contacts of the allotting switch. Other arrangements are described, including some in which one or more of the finders are overflow finders and are allotted only when all the regular finders are busy. Specification 547,368 is referred to.