Patent GB-374977-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q3/00
Filed: 1931-03-23
Abstract: 374,977. Automatic exchange systems. AUTOMATIC ELECTRIC CO., Ltd., Strowger Works, Liverpool, and JACKSON, H., 5, Maple Grove, St. Helens, Lancashire. March 23, 1931, No. 8923. [Class 40 (iv).] In a small exchange of ten lines employing relays only, the non-numerical selecting arrangements comprise a relay counting-chain P .. U, Fig. 2, testing each line in turn in the manner of a finder and consisting of six relays only for the ten lines. The numerical selecting arrangements comprise a set of six two-step counting relays AP.. AU, Fig. 3, which also act as the actual connecting relays, each talking- circuit connection including a plurality of counting-relay contacts in series. Two link circuits are provided and if both are busy when a further call is initiated the calling subscriber is given a busy tone through the winding of his line relay. Non-numerical selection. When calling subscriber No. 6 removes his receiver his line relay L connects ground to lead 9 energizing relays ST, K. Relay P is thus connected over lead 13 to the test wire of line 1 which if non-calling or busy will be grounded. Relay P therefore operates connecting relay Q to the test wire of line 2. Relays Q .. T thus energize in succession, each unlocking the previous one in the chain. When line 6 is reached relay U does not energize and after a pause relay ST releases, connecting relay A across the calling loop. The cut-off relay CO energizes in series with the left-hand winding of relay L which remains energized. The circuit of relay K includes back contacts of relays U, T in parallel so that, if a subscriber of late number abandons a call before his line is reached and a subscriber of earlier number initiates a call after his line has been passed, the testing operation will be repeated. Numerical selection. Relay A repeats impulses over lead 18 to the relay chain AP.. AU which operates in known manner each relay being of two-step type and each, except AU, being unlocked by the next in the chain. At the end of the digit relay E relapses and if the called line is free relay H operates over lead 16 in series with the called subscriber's line and cut-off relays and connects up ringing current over relay F in the usual manner. If the called line is busy relay G is locked and busy tone is supplied from lead 19. Permanent loop. Thermostat TT energized over lead 9 completes after an interval a circuit for relay TR which releases relay K. Relay CO is locked over its left-hand winding and the loop, disconnecting relay L. Relay TR locks itself and can only be released (by shortcircuiting) when the thermostat has cooled down so that premature operation cannot take place for a succeeding call. Link circuits busy. When lead 9 is grounded, two-step relay SW operates partially and when the calling line is found operates fully extending lead 9 to the second link circuit over lead 21. If both are busy when a subscriber calls, busy tone is supplied through the right-hand winding of relay L. Dialling-, ringing-, and busy-tone supply, Fig. 4. When a call is initiated, lead 10 is grounded and relay BR generates busy-tone by intermittently short-circuiting resistance 27. A vibrator V interacting with relay PA generates ringing current in transformer RT, and relay PA at contact pa 2 produces dialling-tone in lead 24. A direct-current relay AC shunted by a rectifier responds to the ringing-current to close a local circuit for vibrator V over a resistance.