GB476391A - Improvements in and relating to electric incandescent lamps - Google Patents

Abstract

476,391. Electric incandescent lamps. BRITISH THOMSON-HOUSTON CO., Ltd. June 11, 1936, No. 16339. Convention date, June 15, 1935. [Class 39 (ii)] In an electric incandescent lamp provided with a filament containing a carbide of the group consisting of tantalum, hafnium, zirconium, and niobium, chemical dissociation of the carbide during operation of the lamp is prevented by providing a carbon-bearing medium which is decomposable at or about the operating temperature of the filament to deposit carbon thereon, and a source of uncombined carbon serving to regenerate the carbon medium. In the example the carbide filament 4 is mounted in a bulb 1 which may be filled with argon at a pressure within the range of 0À5-2À0 atmospheres. The source of uncombined carbon for preventing chemical dissociation of the filament consists of a deposit of lamp-black, or soot on the filament leading- in wires 5, 6 and on the stem 2 and press 3 of the bulb. To facilitate the transfer of carbon from the source of supply to the filament the bulb contains a quantity of carbon disulphide, which is decomposed at the temperature at which the filament operates. Alternatively other vaporizable materials of similar characteristics, such as carbon tetrachloride, may be used. In a modification the source of uncombined carbon consists of a solid body of carbonaceous, material supported adjacent to the filament, while in a further modification a carbonaceous filament with its own separate leading-in wires is suspended below the refractory carbide filament. The resistance of the carbonaceous filament is such that its temperature is lower than that of the main filament; for example with a main filament of tungsten carbide at a temperature of about 3000‹ C., the temperature of the auxiliary filament will be about 800‹ C. The carbide filaments may consist wholly or partly of a carbide, or of a mixture of two or more carbides, for example four parts of tantalum carbide to one part of either hafnium or zirconium carbide. The filaments may be formed by known methods, a preferred method comprising the mixing of the oxide of tantalum, or a corresponding metal, with a cellulose binder to obtain a plastic mass which is extruded through a fine orifice. The resulting filament is carbidized by heating with free carbon in a reducing atmosphere.