Patent GB-431946-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q3/00
Filed: 1934-01-18
Abstract: 431,946. Automatic exchange systems. SIEMENS & HALSKE AKT.-GES., Siemensstadt, Berlin. Jan. 18, 1934, No. 1839. Convention date, Jan. 18, 1933. [Class 40 (iv)] Subscribers' lines are connected to a common line relay over individual gas discharge tubes or rectifiers, e.g. dry rectifiers. Where the common relay serves a large number of lines the circuit over which it is energized includes, in addition to rectifiers individual to the lines, gas discharge tubes common to subgroups of lines, Fig. 4 (not shown). False operation due to the cumulative effect of the leakages of the whole group is thus prevented. The invention is described with reference to call-finder systems. First system, Fig. 1. A calling subscriber after removing his receiver, temporarily, grounds his upper talking wire, and the common line relay An, which is thus operated over the discharge tube, e.g. G10 (or rectifier) and then released, gives an impulse to the back-stroke driving magnet DI, DII &c., of the first idle finder. The latter (say ASI) on going off normal extends the starting circuit to the next finder at its contacts 29 wI and at 23wI connects up FI. The switch then advances by interaction between FI and the driving magnet until the calling line is reached whereupon impulse relay A and release relay B pull up over the loop and FI is cut off. Relay CI, which disconnects wipers aI, bI to prevent interference with busy lines, receives an impulse at each operation of the magnet and holds over the wiper cI when busy lines are encountered. Second system, Fig. 2, with means for dealing with simultaneous calls. In this case the removal of the receiver momentarily grounds the starting electrode II of the discharge tube, e.g. G10 and the common line relay An remains energized owing to the persistence of the discharge between electrodes I, IV. The finder magnet (say) DI receives impulses from an interrupter RU, the circuit being maintained over off-normal contacts independently of An and being broken when the calling line is reached by BI which pulls up over the loop in series with AI. CI operates as in the first system to prevent interference with busy lines. BI also, over a fourth wiper on the finder, momentarily grounds the extinguishing electrode III of the tube G10. After a short interval a relay UI (not shown), extends the starting circuit and, if another call is waiting and An therefore has not been released by the cessation of the discharge in G10, the second call is extended in a similar manner. The tube shown can be replaced by several two-electrode tubes in series but not by rectifiers. Third system, Fig. 3, in which there is no ground at the substation. Separate discharge tubes such as Ga1, Gb1 are connected in each leg of the line and when the loop is completed at the sub-station, both tubes break down and the common relay An is energized from a high-voltage source such as the supply mains or the exchange battery and an additional source BB in series. The finder hunts as in the other systems but the relay C is omitted since the test circuit over the loop includes a discharge tube such as GI which is unable to strike until it finds the high voltage of a calling line. BI on operating shortcircuits GI and puts momentary full ground and subsequently high-resistance ground on the middle electrode of Gb1. This extinguishes Ga1 and stops the discharge to electrode 3 of Gb1. If all finders are busy, a tone signal is given with the aid of an additional winding on An. A pair of two-electrode tubes in series or a single such tube in series with a resistance may be used in place of Gb1. The discharge tubes in this system may be replaced by rectifiers. Relay BI may have a high resistance winding which is shunted when the relay pulls up.