Patent GB-229756-A (GB)
Title: Improvements relating to switching keys for telephone systems
IPC: H01H19/62
Filed: 1923-11-28
Abstract: 229,756. Siemens Bros. & Co., Ltd., Goodwin, W. E., and Garrod, E. R. Nov. 28, 1923. Automatic and manual exchange systems. - An extension key, especially useful in an automatic exchange, for controlling the connection of an exchange line to a main or extension instrument, provides for a sequence of switching operations so that the metallic continuity of a holding bridge across the exchange line is not interrupted when the exchange line is transferred from the main to the extension instrument. Construction of key. A spindle 5, Fig. 5, carrying four cams 10, 20, 30, 40 is rotatable in a fixed block 6 by means of an operating handle 7, to which is attached a pointer 8 moving over a scale marked with four positions, as illustrated in Fig. 1. A knob 12, Figs. 5 and 6, mounted on a spring 11, rides on the periphery of each cam 10, 20, 30, 40, and is adapted to operate its respective spring set 1 - - 4 through an insulating block 13. The cams 10 - - 40 are shaped as shown in Fig. 3, and as the handle 7 is turned through 360‹, close the spring sets as indicated in the chart shown in Fig. 4. In the last-mentioned Figure, the humps on the contours represent the closed positions of the corresponding spring sets; the intermediate position of the set 2 is a neutral position; whilst the intermediate position of the set 3 is a make-before-break position. In a modification, the spring sets are operated by spring plungers, the upper ends of which ride on concentric cam projections formed upon a disc rotated by the handle 7. Circuit arrangements, Fig. 2. As shown the key switch is in position 1 in which the main instrument, connected to the leads 16, 17, is extended to the exchange line 14, 15 over the lower contacts of the spring sets 2, 4. In this position a bell 22 for signalling the main attendant is connected across the extension leads 18, 19. If the main subscriber desires to hold the exchange line and speak to the extension subscriber, he turns the switch clockwise to position 3, i.e. through 90‹. From an inspection of Fig. 4 it is seen that springs 1 are first operated and a bridge, including one winding of the bell 22 is prepared for the exchange line 14, 15. The makebefore-break springs 3 shortly afterwards connect up this bridge and a little later the spring set 2 is actuated and connects the main instrument line 16, 17 to the extension line 18, 19. The extension subscriber having been signalled by a magneto generator, the exchange line 14, 15 may be connected to the extension line by rotating the key a further 90‹ to position 4. At the commencement of this movement the bridge is also connected across the exchange line 14, 15 at springs 4 to replace the connection at springs 3 which shortly after restore, and connect the extension line 18, 19 to the exchange line 14, 15 in series with an indicator 23, which is subsequently used to give the disconnect signal at the main instrument. Finally, springs 1, 2 are restored and the bridge removed. The main attendant may listen in on the connection by connecting the leads 24, 25 to the exchange leads 14, 15. If the main subscriber requires connection with the extension when he is not in speaking connection with the exchange, the key switch is turned anti-clockwise through 90‹ to position 2, thereby first actuating the springs 3 to connect the bell 22 across the exchange line 14, 15, and then the springs 2 to connect the main instrument leads 16, 17 to the extension leads 18, 19. Should the main instrument receiver be removed, and it is then noticed that the switch is in position 4, a false impulse is prevented from being sent to the exchange when it is returned clockwise to position 1, since the springs 2 are in neutral position when the springs 4 revert to normal. The spring-set 3 is of the make-before-break type described in Specification 209,531, [Class 38 (v), Electric switches &c.], in order that the exchange can ring the subscriber even if the switch-key is inadvertently left in an intermediate position.