Patent GB-814952-A (GB)
Abstract: 814,952. Cold-cathode tubes. WESTERN ELECTRIC CO. Inc. May 3, 1957 [May 9, 1956(2)], No. 14171/57. Class 39(1). A negative resistance characteristic over a wide range of currents in the abnormal discharge region is obtained in a gaseous discharge device having an anode and a cathode of high density electron emission by interposing between the cathode emissive surface and the anode a physical barrier having an orifice therein which constricts the discharge current flow between the cathode and anode, and suitably adjusting the size of the orifice, the gas pressure and the anode-cathode gap. The tube shown in Fig. 13, comprises an envelope 20 preferably of glass filled with neon at a pressure of 95 mm. of mercury. A lead wire22 of Kovar (Registered Trade Mark) sealed through the envelope is butt welded to a molybdenum wire 38 forming the cathode and having a flat end 39. A molybdenum cap 103 having a small central aperture is crimped to the wire 38 and preferably has an external coating of aluminium oxide. A molybdenum anode wire 31 is welded to the flattened end 109 of a length of Kovar tubing 108 so as to be offset from the axis of the wire 38 and the aperture in the cap 103. The tubing 108 serves as an exhaust stem and is welded flat at the end 111 to form the final vacuum seal. A barium or other getter 23 is mounted on a U-shaped support 112 secured by a wire 113 to the lead wire 22 in a position such that the getter flash 37 is deposited on the inner wall of the envelope 20 adjacent to the anode-cathode gap, and makes contact with a Kovar wire terminal 36 seated through the envelope 20. The glass-to-metal seals at the ends of the envelope are masked from the getter flash by nickel eyelets 114 and 115 secured to the lead 22 and tubing 108 respectively. The terminal 36 may be connected to the cathode lead 22 through an external reactor so that the getter flash 37 acts as a photosensitive surface and provides residual ionization in the anode-cathode gap. Alternatively, radio-active material may be employed to provide residual ionization. After sealing off the tube is subjected to an ageing process in which a current of the order of the normal discharge current for the tube is passed for some hundreds of hours. During this process the central portion of the end 39 of the wire 38 is eroded and material is deposited on the outer parts as shown in Fig. 15 to form with the hemispherical end 105 of the cap 103 a cavity of slightly elongated spherical shape. If the size of the orifice 104 is initially incorrect the tube may oscillate and material is also removed or deposited to adjust the orifice to the correct size. The ageing time may be reduced by preforming the end of the wire 38 to a hemispherical cup shape, and the process may be carried out .with pulsed current or in two stages using different current values. In an alternative embodiment the getter 23 is supported on a separate lead sealed through the envelope 20 and the cathode is initially a rod 90, Fig. 10, having an axial cylindrical cavity 91 in the end facing the anode. During ageing the cavity is deformed to the elongated spherical shape shown by the dotted lines. The end face 25 forms a barrier having an orifice 29. Alternative cathode materials include tungsten, tantalum, and columbium, and alternative gas fillings include helium and neon-helium mixtures. Curves are given plotting current against voltage, current against voltage at different gas pressures, negative resistance against current at different gas pressures and at different frequencies when the device is used in a transmission path in signalling or switching networks, negative resistance against reactance at different frequencies and operating currents, resistance against anode-cathode gap at different gas pressures, and negative resistance against reactance for different frequencies and orifice diameters. The cathode support or rod may be provided with heat-radiating fins. Specifications 686,427, 704,108, (Group XXXIX), 789,875 and 789,876 are referred to.