GB309447A - Improvements in or relating to thermionic devices - Google Patents

Abstract

309,447. British Thomson-Houston Co., Ltd., (Assignees of Hull, A. W., and Ruggles, W. A.). April 10, 1928, [Convention date]. Vacuum tubes; low - pressure metallic-vapour apparatus. - Thermionic cathodes are formed with a hollow central support carrying a plurality of vanes, heating means for the support and vanes, and a surrounding member adapted to prevent loss of heat in order to obtain a high thermal efficiency of electron emission. In one example shown in Figs. 1 and 2, a stem 6 supports the cathode structure 7 and a grid 8 which may be formed of punched sheet metal supported by wires 9, 10 attached to a band 11, the wire 10 being extended and connected to a contact 12. The cathode 7 comprises a cylinder 13 of nickel nickel alloys, tungsten, or molybdenum containing thorium oxide and has an internal refractory insulating lining 14, a heater 17, and radial vanes 15 secured to a concentric cylinder 16 which may be polished externally. The ends of the heater 17 which may be of tungsten are connected by welding, riveting, or otherwise to the thickened end wall 18 of the cylinder 13 and a conductor 19 respectively, while the wires 20, 21 supporting the cylinder 16 and the wire 19 are sealed through the stem 6 and connected to contacts 23, 22. Heat conduction is reduced by cutting away the edges of the vanes 15 which may be of nickel or iron. Argon at a pressure of 50 microns or a vapour-yielding material is inserted in the envelope. When mercury is used, a shield 25 is mounted on the stem 6 to prevent heating of the base and the consequent formation of a high vapour pressure. The interior surface of the cathode and the vanes are coated with barium, caesium, cerium, a mixture such as misch metal, or oxides of these metals. A coating of barium carbonate may be applied to a nickel cathode which is then heated to 1300‹C, or thorium oxide may be used when the cathode is of tungsten or molybdenum. The vanes may consist of sheets of metal having bent end portions, Fig. 3, or may have a triangular shape, Fig. 4, or may be V-shaped, Fig. 5, the apex of the V being secured to the internal cylinder. One of more cylinders 30, 31, Fig. 5, may be provided to reduce heat radiation.