Patent GB-475749-A (GB)
Title: Improvements in or relating to automatic telephone systems
IPC: H04Q3/62
Filed: 1936-07-16
Abstract: 475,749. Automatic exchange systems. ERICSSON TELEPHONES, Ltd., and DRYSDALE, L. H. July 16, 1936, No. 19713. [Class 40 (iv)] In an automatic exchange, particularly a small private exchange having a special class of subscriber with preferential service, the ringing-current is characteristic of the class of subscriber making the call, and is determined by a connection over a wiper of the finder. Some of the preferred subscribers may be able to break in on existing connections. The system shown comprises finder-connector links similar to those shown in Specification 402,193, the finders being started and controlled by common apparatus, and the connectors being single-motion switches giving access to lines numbered 1 ..9, 01 .. 09, and 001 .. 004, the 10th and 20th terminals being disconnected. When subscriber S1 calls, relay A connects battery over resistance R1 to the test lead 26 and operates common start relay AS over a rectifier RC1 which prevents the subsequent connection of holding ground to lead 23 from disabling the starting circuit for other subscribers. Relay AS operates a relay SA .. SN in one of the free link circuits, the relay which operates first, SA say, disconnecting all the others. Relay SA completes the stepping circuit for the finder magnet FDM and when the calling line is found, common test relay K operates over wiper fw6 and connects relay B in series with the magnet. Relays B, C, RT operate and relay MSA starts the ringing interrupter I over lead 28. Dialling tone DT is connected up by relay RT and is disconnected at the first step of wiper cw4, but relay C remains operated over its left-hand winding during impulsing. Impulses are repeated over wiper fw5 to connector magnet CDM and at the end of the digit or digits the relapse of relay C connects test relay H over wipers cw4, cw3 to the test lead 34. If the called line is free, relay H operates, locks in series with magnet CDM and connects up the ringing circuit. Ringing is effected by rapid intermittent grounding of the a-wire by interrupter I, the consequent rapid changes of potential at the bell condenser causing ringing of the bell. The ringing circuit is interrupted at different frequencies for the two classes of subscribers by two self-interrupting relay chains BP .. RR and X .. Z, the change-over being effected by a relay DS operated for preferential subscribers by a battery connection over resistance R3 to wiper fw7. When the called subscriber replies, relay AA completes, during a silent interval, a circuit over wiper cw5, lead 31, and lead 30 or 33 for relay RT which trips the ringing. When preferential subscribers are permitted to break in on established connections, the dotted connections are added so that relay RT operates over wiper fw7, leads 33, 31, and front contacts aa1 of the line relay AA of the busy called line, and completes a circuit over wiper cw4 for relay H. Certain of the preferential subscribers may not be permitted to break in, in which case the value of resistance R3 is such that relay DS operates but RT does not. Rectifiers RC3, RC4 prevent faulty operation or locking of relays RT, DS, H over the circuit including contacts ds1 which is provided to ensure the full operation of relay DS when a number of circuits are connected in parallel to resistance R3.