GB865928A - Improvements in or relating to sodium vapour electric discharge lamps - Google Patents

Abstract

865,928. Discharge lamps. GENERAL ELECTRIC CO. Ltd. Nov. 13, 1959 [Dec. 17, 1958], No. 40727/58. Class 39(1). [Also in Group XXIII] A low pressure sodium vapour-rare gas discharge lamp comprises an envelope, e.g. straight or U-shaped, formed from substantially circular section tubing having at least one axially straight part, wherein the or each straight part is formed with a plurality of flattened regions of relatively long axial length separated by intermediate regions of relatively short axial length having the full substantially circular cross-section, the flattened regions each having an area of cross-section between one quarter and three quarters, preferably one half, that of the intermediate regions and having in each cross-section of the region a maximum diameter which is not less than, preferably the same as, that of the intermediate regions. The flattened parts of the envelope in each and all of the flattened regions can be parallel. In the construction shown the U-shaped envelope 1 is formed from soda glass with an internal coating of sodium-resistant borate glass. Alternatively the whole of the envelope or the part bounding the discharge space can be formed of borate glass having the percentage composition by weight; B 2 O 3 -25.0, K 2 O-5.5, CaO-9.0, BaO-36.0, and Al 2 O 3 -24.5. The envelope has a filling of sodium sufficient to produce a vapour pressure in operation of much less than 1 mm., usually about 0.01 mm., and neon containing approximately 1.0% krypton and 0.1% xenon at a total pressure of about 14 mms. The envelope is mounted within a glass outer jacket 3. A slotted circular mica disc 9 supports the end of the envelope within the jacket. The disc 9 is held in position by a pair of starting wires 15 which are each connected to an appropriate one of mounting wires 6 connected to electrode leads 8, pass through holes in the disc, and are anchored to a mica tag 16 carried by a spring clip 17 fitting over the end of the envelope as described in Specification 815,782. A cylindrical glass sleeve 12 is fitted closely around each limb of the envelope as described in Specification 801,482 and a metal spring member 13 fits between the limbs at the curved end of the envelope and has flanges 14 which bear against the adjacent ends of the sleeves 12. The member 13 has spring fingers 18 extending into the space between the adjacent ends of the sleeves 12 and the limbs of the envelope. The jacket 3 is evacuated and contains two getter elements 19 attached to the wires 6. The outer surface of the envelope 1 at the curved end can have a heat reflecting coating, e.g. a layer of silver or platinum paint. The flattened regions 20 are formed so that the major axis of each region lies in the plane containing the axes of the limbs of the envelope. The envelope in each flattened region may be flattened on one side only of the tube axis, and in the case of a U-shaped envelope the flattened parts can be formed facing outwards and parallel. The flattening is effected by heating the appropriate regions of the envelope before sealing-off to a high temperature, and pressing the heat-softened regions inwardly between opposed dies whilst the pressure within the envelope is maintained at slightly greater than atmospheric. The lamp is operated with the limbs of the envelope approximately horizontal and with one directly above the other. The outer jacket 3 can be single walled and evacuated, or a double-walled Dewar jacket within which the discharge envelope is detachably mounted with air between the envelope and the jacket.