GB1095712A - Improvements in or relating to electric discharge devices having envelopes of high alumina content material - Google Patents

Abstract

1,095,712. Metal-vapour lamps. GENERAL ELECTRIC CO. Ltd. Jan. 6, 1966 [Jan. 7, 1965], No. 743/65. Heading H1D. An electric discharge device having an envelope 1 of high (>85% by weight) alumina content material includes at least one auxiliary electrode constituted or carried by the inner part of a metal annulus 23 incorporated in the envelope wall in the vicinity of a main electrode of the device. In the high pressure sodium vapour lamp shown the main electrode 7 in the form of a silicated tungsten rod, on which is wound a tungsten coil 8 retaining activating material 9, is brazed by titanium 10 to a niobium end cap 6 to which is also brazed a niobium tag 12. The cap 6 is sealed to an alumina plug 2 by washers 30, 31 and 32 of zirconium, vanadium and titanium respectively which form an alloy when heated under pressure during the sealing operation. Similar seals 24 and 25 (shown in their final state) are located on each side of the niobium annulus 23 which has connecting tags 26 and 27 and has welded thereto a tungsten wire extension 28 forming the auxiliary electrode. The second main electrode 14 is brazed to a molybdenum rod 16 provided with an evacuating and filling duct 21 and having a shoulder 17 which is brazed to end cap 5 and to which is brazed an exhaust tube 19 of titanium or niobium. The envelope 1 may be of light-transmissive sintered polycrystalline alumina containing 1% by weight of magnesia and the filling may be sodium and a rare gas, e.g. argon at a pressure of 20 mm. of mercury. Preferred dimensions are given, and the lamp is mounted in an outer glass jacket (not shown).