GB561177A - Improvements in and relating to the construction and manufacture of cathodes for electric discharge devices - Google Patents

Abstract

561,177. Discharge lamps; thermionic cathodes. BRITISH THOMSON-HOUSTON CO., Ltd. July 28, 1942, No. 10560. Convention date, July 29, 1941. [Class 39 (i)] [Also in Group XXII] A cathode for an electric discharge device, more particularly a fluorescent lamp, comprises a conductive core wire 15, Fig. 2, surrounded by a fine wire helix 11, the cross-section of which is larger than that of the core wire and which is bounded by curved portions of wire connected by straighter portions. The core wire serves as a heat reservoir to prevent the formation of hot spots at the points where the discharge strikes the wire helix. The points of contact between the helix and the core wire may be spot welded. The helix may be coated with activating material which may also fill the spaces between it and the core wire. A cathode in which the helix is of flattened oval form may be formed by winding a tungsten wire 11 around larger wires 15, 17, of tungsten and molybdenum respectively, as shown in Fig. 5, and then winding the composite structure about a mandrel 18, Fig. 7. After heat-treatment, to set the tungsten wires in their coiled configuration, the wire 17 is dissolved out by treating the structure with a mixture of nitric and sulphuric acids. The mandrel 18 is preferably also of molybdenum so that it may be removed in the same process. In a modified construction the helix is of triangular section, the corners of the triangle being rounded.