Patent GB-686317-A (GB)
Title: Improvements in or relating to electric discharge devices
Inventor: HUBERT-HOUGH GEORGE
IPC: H01J17/00
Filed: 1949-12-02
Abstract: 686,317. Cold-cathode tubes. STANDARD TELEPHONES & CABLES, Ltd. Dec. 2, 1949, No. 31020/49. Class 39(i) A cold cathode gas-filled electric glow discharge tube comprises electrodes adapted and arranged to form a main discharge gap, an auxiliary trigger gap of lower striking voltage than said main discharge gap and a third priming discharge gap, the electrode arrangement being such that, when suitably polarised under operating conditions, the trigger gap electrodes shield the main gap from ionisation products from a continuous discharge at the said third gap, which discharge is of sufficient magnitude substantially to eliminate the statistical delay time in firing of the said trigger gap. In the embodiment shown in Fig. 1, the anode 6 is formed from a sheet of metal whose ends pass through slots in a mica sheet 16 and are bent over at the back. The mica sheet is clamped by means of eyelets 18 to the opposite upturned sides of a pair of channel members 21, one only of which is shown. The cathode 7 is secured to a mica sheet 22 by a metal strip 23 the thickness of which is less than the length of the cathode fall of potential for normal glow discharge, so that the glow is confined to the surface of the cathode facing the anode. The trigger electrode 8 is also mounted on the mica sheet 22 and is shaped to form an enclosure 10 shielded from the field of the anode 6. A sheet of insulating material 27 prevents discharge between the cathode and trigger connections at the rear of insulator 22. The insulators 22 and 27 are attached to the other upturned sides of the channel members 21 by eyelets 30. The trigger surface facing the anode is covered by an insulator 24, in turn covered by a metal plate 26, both of which are mounted in slots in the channel members 21. The metal plate 26 and channel members 21 may be given a positive bias to improve the deionisation time of the tube. The end of the cathode 7 protruding into the trigger enclosure is tapered so that the trigger discharge occurs over a limited well defined area of the cathode. To asist in this, the inside of the trigger electrode may be calorised except for a narrow region opposite the cathode projection. The auxiliary anode 11 and cathode 12 are rivetted to a mica sheet 31 mounted by means of eyelets to rods 35 which are welded to the eyelets 18 and 30, the anode being spaced from the cathode a distance less than the length of the cathode fall of potential for normal glow discharge so that the auxiliary priming discharge is confined to the wall of an aperture 13 in the anode and a beam of electrons is projected into the trigger enclosure 10 through apertures 14 and 15 in insulating members 27 and 22, but these priming electrons will not emerge past the trigger gap 9 owing to the potential gradient between electrodes 7 and 8. The electrode assemblies are supported by wires which pass through apertures in the lower of two supporting mica discs 3 and 4. The gas filling comprises 92 per cent neon, 7 per cent hydrogen and 1 per cent argon at a pressure of 80 mm. of mercury. In an alternative embodiment, Figs. 6 and 7, the main cathode 43 and anode 44 are coaxial cylinders. A metal band 47, secured to the anode, supports a mica insulator 48 to which extensions 50 of the sleeve anode 49 of the auxiliary priming discharge gap are rivetted. The auxiliary cathode 52 is mounted as a push fit between two mica insulators 53 within the anode 49, connection to the anode being made by a lead 51 which is attached to one of the extensions 50. The trigger electrode 54 and cathode 43 are aligned by means of three insulating discs 56, 57 and 58 eyeletted together, disc 57 being of larger diameter and separating the two electrodes. Cathode 43 is eyeletted to a mica insulator 59 seated inside the anode 44 on a spring collar 60, projections 61 on the anode being bent over to lock the assembly. The trigger gap is formed between opposing portions of trigger electrode 54 and cathode 43 through slots 64 and 63 cut in insulators 56 and 57. Part of the edge of the trigger electrode is cut away, leaving a tongue 65 which is slightly bent over to enter the slot 64 and forms the discharge surface of the trigger electrode. A hole 66 in the anode 44 enables conditions at the trigger and priming gaps to be observed. In a further embodiment, Fig. 9, the auxiliary priming gap assembly 70 is offset from the tube axis, and the main anode 66 is mounted inside the cathode 67. The trigger electrode 69 is a metallic strip.