GB302307A - Improvements relating to electron discharge apparatus - Google Patents

Abstract

302,307. British Thomson-Houston Co., Ltd., (Assignees of Thompson, H. C.). Dec. 14, 1927, [Convention date]. Vacuum tubes. - An electron discharge device comprises an electronemitting cathode and at least two other electrodes one of which is provided with a coating of a compound of a metal having alkaline properties, e.g. borates of potassium, lithium, sodium, barium, and magnesium, and oxides of barium, strontium, and calcium, to increase its efficiency as an emitter of impact electrons. A bead of the compound may be formed on a grid support and evaporated off into a plate by electron bombardment of the grid, or the compound may be mixed with a thin varnish and applied to the electrode and heated during evacuation of the device. If a coating of barium oxide is required the peroxide may be applied and converted by heat. The impact emission is improved by supplying to the space adjacent the coated surface a metal vapour having alkaline properties, e.g. magnesium or calcium. This may be done by applying a quantity 9 of metal in paste form to a grid support and heating during evacuation. The device shown comprises a filament cathode 1, surrounded by a control grid 2., which is surrounded in turn by a screening grid 3, an anode 4, and another screening grid 5, the grids 3, 5 being electrically connected together. The screening grids being operated at a fixed positive potential absorb a part of the electron stream that should proceed to the anode, but the coating causes them to yield an equal number of secondary electrons. The coatings may be used in negative-resistance devices such as described in Specifications 15555/15 and 150,359.