Patent GB-674862-A (GB)
Title: Apparatus for receiving signals over telephone lines by changing of resistances
IPC: H04Q1/38
Filed: 1948-12-10
Abstract: 674,862. Automatic exchange systems. TELEFONAKTIEBOLAGET L. M. ERICSSON. Dec. 10, 1948, No. 32011/48. [A Specification was laid open to inspection under Sect. 91 of the Acts, June 13, 1949.] Class 40 (iv). In a system in which digits are signalled by a subscriber to an exchange by connecting different resistances to line at the substation. compensation for the resistance of the subscriber's line is effected by first charging a condenser connected to the grid of a valve whose anode circuit contains a winding of a test relay in a bridge or like circuit used for measuring the loop resistance. When the calling line La, Lb is associated with the register R, S1 brings up R6 and completes a circuit from resistance r2 through the subscriber's loop including an adjustable resistance m, coil D and resistances r11 ... r13. R6 completes a circuit for striking an arc in the tube G2. Condenser C is originally charged to a large negative potential so that G1 is cutoff and the field from the magnet N directs the arc between the electrodes D and F thereby producing a conductive path between them. The condenser C charges over this path, thereby increasing the anode current of G1 to cause the beam to move away from between the electrodes D, F. R6 brings up R4, R3, R5 and R7 in due course, the last disconnecting the electrode F and opening the charging circuit for C which then retains its charge, to act as compensation for the line resistance, until the release of the register.- R7 also connects up dial tone over coil D. The number is signalled by increasing the resistance of m to a predetermined value by means of a push-button. The resulting drop in potential over r2 causes the beam to be deflected to the space between electrodes D and J completing a circuit for the relay chain R11 . . . R13 which successively cuts resistances r11 ... r13 out of the loop to compensate for the increase in m until the beam is restored to the middle position. Resistance is cut out thereby in coarse steps and if the beam overshoots the middle position it passes between electrodes D, E which causes relays R21 ... R23 to reintroduce resistance in finer steps to restore the beam to its middle position. Such of the relays that have operated lock up and a contact tree thereof transfers the digit to the register during the release time of R3 which is de-energized by R4 shortly after R11 operates. When the push button is released the beam moves to the space between electrodes D and E to bring up R2 which releases R11 ... R13, R21 ... R23. Relay R11 operates R4, R3, R5 which releases R2 and the circuit is ready to receive the next digit. When all the digits have been received and the call set up the register applies potential to wire t2 to bring up S3 and so release the register. The battery feed over the line is thereupon reversed so that it is rejected by resistance m and accepted by the telephone set M due to the inclusion of the rectifiers Re1, Re2. If the subscriber hangs up without completing the number-sending, the arc is still directed between the electrodes- D and J even when R11 ... R13 have operated, so that R1 comes up and releases R3 ... R5 to complete a circuit for S3 which effects the release of the register. The circuit is also adapted to receive dial impulses. On each dial break events occur as for a subscriber hanging up with the exeption that slow-release relay R5 holds up and R1 repeats the digits to the register over wire t1. In a modification, Fig. 2 (not shown), the magnetically controlled electron beam is replaced by a polarized relay in the diagonal of a bridge circuit of which the line loop forms an arm, the relay being capable of taking up two operated positions and a neutral position. Digits are sent by raising or lowering the loop resistance from a standard value and resistances are inserted or removed from the balancing arm by relays controlled by the polarized relay. Specification 666,123 is referred to. The Specification as open to inspection under Sect. 91, describes an arrangement similar to that of Fig. 2 (not shown), but wherein line resistance compensation is effected by the insertion of resistances in the line loop under control of the polarized relay to bring the total resistance up to a standard value. This subject-matter does not appear in the Specification as accepted.