Cathode heaters for thermionic valves

926,237. Cathode heaters. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Jan. 16, 1962 [Feb. 13, 1961], No. 5276/61. Class 39 (3). During the manufacture of a cathode heater, the heater wire is coated with alumina and heated to a temperature approaching its normal operating temperature for a period of several minutes in an oxidising atmosphere (air or oxygen) at a pressure of the order of 3 X 10-4 mm. mercury, whereby the subsequent blackening of the alumina in use is prevented. Preferably, the heating is carried out by passing the normal working current through the wire when mounted within the cathode in the valve envelope, outgassing at about 10-6 mm. mercury being carried out before and after the oxidizing treatment.