Patent GB-274033-A (GB)
Title: Improvements in and relating to telephone systems
IPC: H04Q3/00
Filed: 1927-03-12
Abstract: 274,033. Associated Telephone & Telegraph Co. July 6, 1926, [Convention date]. Automatic exchange systems. - Subscribers' lines have no individual equipment and are tested by a constantly rotating switch LE which on encountering a calling line delivers as many impulses as are necessary to a pre-selected nonormal call-finder F. Several switches LE controlling different groups of call finders may beassociated with the same group of lines so as to test the lines more frequently and to provide an uninterrupted service in case of failure of one of the common equipments. Construction of switches. The testing switch, Fig. 4, resembles a line switch but is driven continuously through gearing and, to reduce wear, the slip rings and brushes (e.g. 312, 307) are arranged between two wipers instead of between the members of one wiper. The wipers are arranged in one plane and the terminals are pro. portioned and arranged as shown in Fig. 7 so that the wipers become operative in succession and are operative for different lengths of time. The finder, Fig. 5, which is otherwise like a line-switch has an additional wiper 32 loosely mounted on the shaft and normally held by a detent 502 engaging a ratchet 508. When the main wipers are driven, a spring 507 between them and wiper 32 is tensioned, and when the magnet 10 is energized, the detent is disengaged and the wiper 32 flies round into alignment with the main wipers where it is stopped by the engagement of members 506, 509 carried respectively by the main wipers and wiper 32. Call-finder. When the wipers 115, 119 of the testing switch encounter a calling line, relay 103 pulls up over the loop, energizes relay 105, and locks itself in series with relay 102 which disconnects the line wipers 115, 119. Relay 105 energizes magnet 101 of the back-stroke allotting switch FA and connects ground over resistance 129 to the finder switching relay 11 which pulls up its light armature 34. If the finder is already on the calling line, wiper 116 on reaching its terminal causes the finder to switch through as described below. But if not, wiper 118 which makes contact an instant later energizes the backstroke finder magnet 75 and relay 107 over parallel interrupter contacts of the magnet. Relay 107 completes an independent circuit for itself and the magnet to ensure full energization of the latter. The finder thus advances its main wipers in the direction of the arrow (i.e. in the opposite direction to the testing switch) one step for saoh terminal encountered by wiper 118 until the wiper 116 makes contact in the set of the line on which the finder originally stood and the finder is therefore on the calling line. A circuit is then made over the loose wiper 32 for relay 106 which holds up relay 105, energizes relay 104 which unlocks relay 103 but holds relay 102, and shunts resistance 129 to cause relay 11 to switch through, and to energize momentarily the magnet 10. The loose wiper 32 then flies round into alignment with the others. When wiper 116 leaves the terminal, relay 106 releases the remaining relays on the common equipment, allowing the allotting switch to move its wipers one step and completing a self-interrrpter circuit in which it takes further steps if necessary. Relay 104 which is energized over a branch of the latter circuit energizes relay 102 to disconnect the line wipers. Relay 102 is energized also over wiper 117 just before the line wipers encounter a busy line and is held energized until they have passed the line to prevent interference and unnecessary operation of the finders. If a connector seizes a calling line while the finder is in motion, the latter is unable to switch through since, relay 11 is short circuited over its light armature 34 as soon as wiper 31 reaches the grounded private terminal. Connector H. This differs from standard practice in that since the test terminal of an idle line is dead, the initial circuit for the switching relay 20, which has only one winding, is made locally on the relapse of the parallel relay 19 over a back contact of the busy relay 18 and rotary springs 92.