Patent GB-650890-A (GB)
Title: Improvements in or relating to electric discharge tubes
Inventor: REEVES ALEC HARLEY
IPC: H01J17/48
Filed: 1948-01-22
Abstract: 650,890. Automatic exchange switches. STANDARD TELEPHONES & CABLES, Ltd. Jan. 22, 1948, No. 2004. [Class 40 (iv)] [Also in Group XL (a)] A selector circuit comprises a cold cathode sequence discharge tube (see Group XL (a)) of the type described in Specification 639,827, [Group XL (a)], having a first and a second discharge array, each comprising a plurality of cathodes and a co-operating anode or anodes, the said tube being characterized in that corresponding gaps of the respective arrays are so constructed that a discharge sequence may operate along the first array independently of discharges at the second array, while discharge sequences at the second array commence at that one of its discharge gaps determined by the number of gaps of the said first array which have been sequentially fired. The tube 1 comprises a dialling array 2 and a counting array 3, each comprising cathodes 5, 6, 7, 8 and an anode 4. A separate cathode 9 forms a priming discharge gap with anode 4<SP>1</SP> in order that cathodes 5, 5<SP>1</SP> shall be the first to fire in the respective arrays. A train of positive dialling pulses applied to the primary terminals of transformer 16 will cause the gaps of the dialling array to discharge in sequence, a discharge once established being maintained indefinitely by the battery 15. Repeated trains of pulses 19 comprising a series of positive pulses followed by a negative pulse are applied to terminal 11. The positive pulses, which may be modulated in time-position or width and are each associated with a definite speech channel, set up discharges at the gaps of the counting array 3 which are maintained by the battery 10 until the negative pulse takes anode 4<SP>1</SP> below the maintaining voltage for sufficient time to extinguish all discharges at this array. Thus, if two dialling pulses are applied to transformer 16, cathodes 5 and 6 fire and remain glowing, so priming the corresponding gaps 5<SP>1</SP> and 6<SP>1</SP> of the counting array. The first pulse of the train 19 will then fire cathodes 5<SP>1</SP> and 6<SP>1</SP> simultaneously, the second pulse will fire cathode 7<SP>1</SP> and the third pulse will fire cathode 8<SP>1</SP>, whereupon a pulse appears at the secondary terminals of transformer 13. The fourth and last positive pulse will have no effect, as all the cathodes of the counting array are already glowing. The negative pulse then extinguishes the discharges and the sequence is repeated. In general, if the tube has m gaps in the dialling and counting arrays, and n dialling pulses are applied to the dialling array, pulse number (m - - n+ 1) will appear at the output. Moreover, if through interference or other temporary cause, the counting sequence is upset in any one train, it will be corrected for the next train, because the selecting means-the dialling pulses-sets up a permanent selecting condition. The necessary unidirectional coupling between the gaps of the two arrays, together with the suppression of cross coupling whereby a discharge might occur between a cathode of one array and the anode of the other array, is achieved by special shaping and positioning of the electrodes or by interposing an insulating screen between the gaps of the two arrays while leaving a direct path between the cathodes so that ionization coupling may take place (see Group XL (a)), and the unidirectional coupling is assisted by biassing the cathodes of array 2 positively with respect to the cathodes of array 3 by a bias supply 15<SP>1</SP> connected to a tapping point on battery 15. In an alternative arrangement, Fig. 11, using electronic as opposed to ionic coupling, the dialling anode comprises a plate 46 having a series of apertures 49 between the dialling cathodes 42 and the counting cathodes 43, the last of which is connected to an output transformer (not shown). The tube has a common counting anode 41, connected to the cathode of a cathode follower valve 54, the anode of which is connected to a polarizing source 55 and the grid supplied with a pulse voltage of waveform similar to that shown at 56, which represents the waveform at anode 41. The dialling anode plate 46 is kept at a steady positive potential V2 by battery 57. Source 60 serves as a maintaining battery for the dialling array and provides a constant mean voltage V3 between anode 46 and the lead 59 which is connected to the remaining dialling cathode resistances 58. Dialling pulses are applied to the primary terminals 62 of transformer 61. Anode 41 is maintained by the cathode current of valve 54 at a potential V1 which is increased to V1+P during the counting pulses. Individual priming discharge gaps may be provided for both arrays. The pulse amplitude P is then adjusted so that the counting array operates normally without discharges occurring at the dialling array. The potentials V1 or V2 are arranged to be such that in the absence of counting pulses no current is drawn from the dialling cathodes through the apertures 49 when the dialling gaps are discharging, but during the counting pulses the increased potential of anode 41 is sufficient to draw current through any aperture 49 opposite a discharging dialling cathode. A stream of electrons then passes between cathode 42 and anode 41 and ionizes the gap 41-43. Thus, at the first pulse of train 56, those counting gaps opposite discharging dialling gaps will fire simultaneously, and the operation of the device is otherwise similar to that of the arrangement shown in Fig. 1.