Patent GB-739047-A (GB)
Title: Improvements in high pressure metal vapour electric discharge lamps
IPC: H01J61/073
Filed: 1951-08-09
Abstract: 739,047. Discharge lamps. GENERAL ELECTRIC CO., Ltd., and RIGDEN, S. A. R. Aug. 1, 1952 [Aug. 9, 1951], No. 18798/51. Class 39 (1). A high-pressure metal vapour discharge lamp of the type comprising within an approximately spherical sealed quartz envelope at least one pair of main electrodes for the passage of a discharge which provides the main source of light from the lamp, and a filling of metal together with a small quantity of rare gas such as argon for facilitating starting, each of the main electrodes which acts as a cathode during normal operation being of refractory metal and arranged to be heated only by the discharge, has the inner end of at least one main electrode in the form of a block of tantalum having a hollow interior containing activating material, the block being pierced by one or more passages each providing a constricted outlet from the interior and through which activating material can diffuse to the terminal surface of the electrode. In the construction shown in Figs. 1 and 2 of the drawings accompanying the Provisional Specification a mercury-cadmium filled high-pressure lamp adapted to dissipate 2.5 kilowatts in normal operation comprises a spheroidal quartz bulb 1 of longer internal diameter 61 mms. and shorter internal diameter 56 mms. provided at opposite ends of its longer diameter with tubular sealing stems 2, 3 through which pass molybdenum or tungsten rods 4, 5, each of diameter 4 mms., the sealing stems being closed at their outer ends by lead-in seals 6, 7. Each main electrode may consist entirely of a, tantalum block such as a rod, suitably hollowed at its inner end. The bulb 1 is provided with a filling of argon gas at a pressure of 30 mms. of mercury and a globule of 0.47 grs. of mercury together with 0.47 grs. of cadmium. Each of the molybdenum rods 4, 5 is provided at its inner end 8, 9 with a hollow cylindrical tantalum block 1.0, 11 having an external diameter of 6 mms., an internal diameter of 3.5 mms., and an overall length of 15 mms. The terminal surfaces 12, 13 of the blocks are spaced 15 mms. apart and each surface is provided with a hole 14, 15, having a diameter of 1 mm. and passing through the block between the terminal surface 1.2 and the inner surface 12<SP>1</SP>, the distance between the surfaces 12 and 12<SP>1</SP> being 2.5 mms. It is stated that the tantalum inner end of each main electrode acts as a selective getter during the initial ageing of the lamp, absorbing gaseous impurities without appreciably affecting the metal or rare gas filling. Each of the rods 4, 5 is provided at its end 8, 9 with a screw-thread 16 co-operating with a similar screw-thread 17 on the block 10, and pellets of activating material 19 are contained within the hollow blocks 10, 11 and held in position by rods 4, 5. The activating material may consist of sintered alkaline earth oxides or thoria or may be manufactured in accordance with Specification 594,059 from a mixture having the following proportions: barium carbonate 2.5, calcium carbonate 1.0, thorium oxide 2.5, together with a quantity of tungsten powder of weight equivalent to 3 per cent of the total weight of the above mixture. For reducing blackening of the envelope 1 in operation, tungsten coils 21, 22 are provided on the rods 4, 5 so that the ends 23, 24 of the coils abut the adjacent ends of the blocks 10, 11, and the other ends 23<SP>1</SP>, 24<SP>1</SP> abut the adjacent ends of the sealing stems 2, 3. Specification 626,303 also is referred to.