Patent GB-278787-A (GB)
Title: Improvements in or relating to thermionic cathodes
IPC: H01J9/08
Filed: 1926-07-07
Abstract: 278,787. Robinson, E. Y., and Metropolitan-Vickers Electrical Co., Ltd. July 7, 1926. Vacuum tubes.-In making equipotential electrodes for vacuum tubes, of the kind in which a conducting member forming the electrode proper is heated by an electrically heated member such as a filament of refractory metal, the filament is covered wholly or in parts along its length all around its peripheral surface with refractory insulating material directly applied by moulding around the filament or by coating the filament by dipping, spraying &c. with a suspension, sludge or paint consisting of the insulating material and a vehicle or solvent. The insulating material is subsequently sintered, fused, or vitrified and may comprise kaolin with or without felspar, lime or other material to facilitate sintering and increase the strength of the material. Clay slip may be used. A straight tungsten or molybdenum filament may be drawn through a paste of kaolin and water and the coating dried naturally or in a gas flame or in a muffle at 800‹ C. or first naturally and then in a muffle. Two or more coats are applied and the coats are then vitrified by heating to 1500‹ C. in an oven or flame or by passing current through the filament in hydrogen or a vacuum. A metal coat forming the equipdtential surface is then applied either as a separate nickel tube or as nickel or platinum foil or platinum strip wrapped around the heater or by chemical or electrical deposition; nickel may be sprayed on or platinum applied by repeatedly dipping in platinum chloride solution and heating, with subsequent nickel plating if desired. An oxide coat may be applied to the metal by dipping it into or painting it with a suspension of barium and strontium carbonates in water and heating in carbon dioxide, the carbonates being subsequently converted to oxides. The diameter of the completed electrode may be less than 1 mm. A helical filament with leads fitting into its ends may be similarly treated. In making a non- inductive heater, a straight filament is coated by dipping and bent into U-form with the limbs touching before the coat is vitrified, a metal sheet 3, Fig. 12, being wrapped around the coated limbs 1 of the filament dinectly or after insulation has been moulded into rectangular form about the limbs 1 in a mould with colza oil to prevent sticking. In a modification, Fig. 7, a further insulating coat 2<a> is sprayed on to the coats 2 oa the limbs 1 and metal 3 is deposited thereon and over a lead at one end, the insulation being firedl before or after the application of the metal. The coat 2 may be scraped from the centre of the filament before bending and a U- or V-shaped nickel member 8, Fig. 15, may be welded to the end of the filament 1 before the coat 2 is applied. The filament may be held in tension during vitrification, of the coating. In a thermionic valve with an equipotential cathode so formed, the anode 18, Fig. 21, is welded to a wire 13 and the grid 19 comprises a tungsten helix supported by a wire preferably as described in Specification 231,225, the wire 21 engaging a socket 17, Fig. 20, and the wires 13, 21 being steadied by wires 22, 23 in a glass bead 24. The filament 1, has leads 14, 15 welded to the arms of the member 8 after cutting the member 8 through and is tensioned by a hook 25 engaging its uncoated loop, the hook being carried by a spring 26 supported. from a bead 28. The cathode tube 20 fits the filament coating slidably and is fixed at one point thereon by a welded lead 9. Specifications 209.415 and 229,654 are referred to.