GB294168A - Improvements in and relating to thermionic apparatus - Google Patents

Abstract

294,168. British Thomson-Houston Co., Ltd., (Assignees of Schmidt, A.). July 19, 1927, [Convention date]. Vacuum tubes; low-pressure metallic-vapour apparatus. - Relates to thermionic apparatus having a cathode-heating element and an exciting electrode and consists in (1) having means connected between the heating-element and the exciting electrode for limiting the starting current of the heater, and which may comprise an impedance device for applying to the starting electrode a potential which is dependent on the current of the heating-element, and (2) in the case of a rectifier, filling the envelope with gas or vapour at a pressure below that at which the discharge becomes arc-like. The filling may be of mercury vapour or rare gases such as argon and neon. The cathode 6 may be coated with a highly-emissive material, e.g. barium or other alkaline-earth oxides, or rare-earth metals such as cerium. Current is supplied to the cathode heating-element 5 by a secondary winding 13 on a high-reactance transformer 10 provided with a three-legged core, and to the exciting electrodes 8 by a secondary winding 14 the centre of which is connected either to a centre tapping of the winding 13, as shown, or directly to the cathode 6. The current supplied to the heating-element before it has become heated is limited by the magnetic shunt effect of the cone leg on which the winding 14 is wound, and the extra flux in this leg when the current in the winding 13 is high at starting causes a comparatively high potential to be applied to the exciting electrodes. The middle leg of the transformer 10 may be provided with an air-gap, as shown in broken lines, to produce a sufficiently high reactance.