Patent GB-689962-A (GB)
Title: Improvements in or relating to high pressure electric discharge lamps
IPC: H01J61/073
Filed: 1948-04-02
Abstract: 689,962. Electric discharge lamps. GENERAL ELECTRIC CO., Ltd., EDWARDS, E. F., FRANCIS, V. J., ISAACS, G. G., and SAMUELS, D. E. J. March 30, 1949 [April 2, 1948; July 26, 1948; July 26, 1948; Jan. 21, 1949], Nos. 9365/48, 19968/48, 19969/48 and 1774/49. Class 39(i). A high pressure metal vapour lamp containing only or mainly mercury and operating at a pressure of one atmosphere or more, preferably three atmospheres or more, such as of the order of five atmospheres, comprises a store of material which when heated evolves the vapour of one or more of the metals lithium, sodium, barium, strontium, the store being so positioned in relation to the discharge path of the lamp that when the lamp is operated from a direct current supply at a suitably high wattage, not less than 200 watts per cm. of the discharge path, the store is so heated by the discharge, and optionally also by additional heating means, that the vapour of one or more of the said metals is evolved and passes into the path of the high pressure discharge at such a rate that there is produced a luminous glow, emanating mainly from the neighbourhood of the cathode, and characteristic of at least one of the metals lithium, sodium, barium, strontium. Filling metals additional to mercury may be cadmium, zinc, thallium; or indium or tellurium according to the first Provisional Specification. Argon may be included to facilitate starting. The ratio of the area of the inner surface of the envelope in square cms. to the square of the distance between the electrodes in cms. is greater than #, or 5 at a wattage loading of not less than 500 watts per cm. of the discharge path, and the amount of metal within the envelope is sufficient to establish a vapour pressure not less than that resulting from one milligram of mercury, or 3.5 milligrams of mercury at a wattage loading of not less than 500 watts per cm. of the discharge path, per c.c. of volume within the envelope. In the construction shown in Fig 1 the spherical quartz envelope 1 has sealed into it at diametrically opposite points a tungsten rod anode 2 with a chamfered end face and a tungsten rod cathode 3 having a tapered portion formed to a cylindrical spike 4. The anode and cathode rods are supported by, and leads to them are brought out through, strip-seal side-stems 5 and 6. The envelope carries also a third strip-seal side-stem 7 which supports, and serves for the supply of current to, a tungsten rod auxiliary electrode 8, the end of which carries a helix of tungsten wire. with turns touching, carrying within it a barium thorate rod and closed at its end near the cathode 3 by a tungsten plug 11. The auxiliary electrode 8, 9, 10, 11 is arranged substantially at right angles to the cathode 3. A few turns of thoriated tungsten wire 12 are wound round the cathode 3 between the end of the plug 11 and the base of the spike 4 to serve as a starting electrode. The lead from the strip-seal 7 is connected to the anode strip-seal 5 through a current-limiting resistor 13. The lamp contains about 250 milligrams of mercury and operates at 500 watts. The construction shown in Fig. 2 is similar to that shown in Fig. ] except that the auxiliary electrode 8, 9, 10, 11 of Fig 1 is replaced by a helix 14 of tungsten wire, with turns just separated, enclosing a rod 15 of barium thorate and supported from the ends 16, 17 of separate molybdenum strip-seals brought out through a side-stem 18. The helix 14 is supported with it axis parallel to the cathode 3 and with the end of the helix approximately level with the base of the spike 4. The starting electrode 12 lies between the helix 14 and the envelope 1. In the construction shown in Fig. 3 the envelope 19 is roughly egg-shaped, consisting of a substantially hemi-spherical portion 20 joined to a conical portion 21. The tungsten anode rod 22 is sealed into the conical portion 21 and the tungsten cathode rod 23 is sealed through the hemi-spherical portion 20, the anode and cathode lying along the axis of the lamp and being supported by, and leads to them being brought out through, strip-seal sidestems 24, 25. The end of the anode 22 is rounded to substantially hemi-spherical shape and the end of the cathode 23 is ground to a conical point. The cathode 23, just beyond the base of its conical end, is cut away axially by a plane parallel to the axis, and in the groove thus formed a rod 27 of barium thorate wrapped in a helix 28 of very fine tungsten wire is held by a helix 29 of tungsten wire wound round the cathode 23 with turns touching. The lamp is adapted to operate at 2,500 watts and contains about 650 milligrams of mercury. The store of material containing the vaporisable additional metal may be mounted on or embedded in an electrode other than the cathode. The store of material may be heated by a spiral of refractory metal, such as tungsten, in which the material is placed. Part of the cathode may be coated with metallic barium, or a stick of a barium compound such as barium silicate may be provided within the heating filament, or barium thorate or oxide may be placed in a groove or hole in one of the electrodes. Sodium silicate and a small rod of soda glass are also referred to. Two auxiliary electrodes, either or both of which are provided with a store of material containing the additional metal, can be used. Dimensions of the envelope, electrodes, etc., and running characteristics of the lamps are given and differ from, those given in the Provisional Specifications. Specifications 1308/04, [Class 39], 104,674, [Class 39(i)], 402,747, 404,865 and 405,605 are referred to.