GB414375A - Indirectly heated cathode - Google Patents

Abstract

414,375. Thermionic cathodes. LOEWE, S., 10, Wiesenweg, Steglitz, Berlin. Feb. 1, 1933, No. 3060. Convention date, Feb. 2, 1932. [Class 39 (i).] An indirectly-heated cathode of the kind in which the emissive surface is prepared by exposing an equipotential surface to metal vapour is so constructed that any emission due to deposit of emissive metal on those parts of the insulating tube not covered by the equipotential surface, and any leakage between the heater filament and the equipotential surface due to such deposit are prevented by means other than those involving coating the ends of the insulating tube with a rough, non-emissive coating. In one construction, screens are so disposed as to prevent deposit on the ends 6 of the insulating tube. In the form shown the equipotential layer consists of two metal tubes 2, 6, which are slipped one over the other, so that the outer tube 2 projects beyond the inner tube 6 at each end, leaving a small pocket 7 into which vapour will not enter. Alternatively the equipotential layer may be in the form of a tube having inwardly projecting resilient ribs. In another construction, the ends of the insulating tube are coated with graphite, which, in addition to fulfilling the objects of the invention, cools the ends of the insulating tube. In another method, the equipotential surface of the cathode of a thermionic valve is shorter than the insulating tube but longer than the grid, which, in turn, is longer than the anode. The activating barium vapour, which may be released from a capsule in the anode, is deposited mainly on the middle of the equipotential surface, whilst any which is deposited on the insulating tube is removed by intensive heating at these points obtained by making the heater coil of closer pitch here than elsewhere. Reference has been directed by the Comptroller to Specification 375,481.