GB951193A - Improvements in low pressure discharge lamp - Google Patents

Abstract

951,193. Discharge lamps. GENERAL ELECTRIC CO. May 2, 1960 [May 11, 1959], No. 15325/60. Heading H1D. An elongated low pressure discharge lamp operated by A.C. contains mercury vapour, an inert starting gas and an electrode structure at each end comprising an activated coiled tungsten filament 13 supported by inlead wires 11, 12, and conductive shields 14, 15 fastened to the inlead wires, the resistance and thermal mass of each filament 13 being such that the filament carries 100% of the discharge current on the cathode half-cycle and less than 50% on the anode half-cycle, the conductive shields carrying the balance of discharge current, and that an electron emission spot temperature in the range of 105‹ to 1200‹ C. is established in the filament when the lamp is supplied with current at the rated voltage. The resistance and thermal mass of each filament 13 are also so chosen that at starting the filament is stabilized at a temperature in the range of 850‹ to 1050‹ C. in a time interval of less than 1 second after the application of the filament heating voltage. The shields 14, 15 are laminar and substantially L-shaped and are made of metal such as iron, tungsten, molybdenum, nickel or nickel-plated iron which can be activated by deposition thereon of activating material, such as barium or barium oxide, from the filament. The area of the shields is sufficiently large to produce a low anode voltage drop when conducting discharge current on the anode half-cycle. The shields are located at a distance from the emission spot surface of the associated filament of not less than approximately 3 mm. Each filament can comprise a double coiled filament 22 provided with a fine triple coiled overwind 21 of tungsten wire coated with alkaline earth oxide activating material. The coiling can be effected on molybdenum mandrels. Dimensions are stated. The inert starting gas can consist of a mixture of argon and neon at a total pressure of between 1.5 and 3 mm., preferably 2 mm., neon being present in a proportion of less than 50%, preferably 25-40% or 35%. The mercury pressure is a few microns. The envelope of the lamp is coated internally with a phosphor which may be treated to remove surface metallic ions, such as antimony and manganese activators, in order to prevent attachment of mercury to the phosphor surface. The envelope wall may be treated to remove alkali materials therefrom to make it a better insulator and to leave less material for reaction with the phosphor surface or to serve as attachment points for mercury. Alternatively a potential can be applied to the envelope wall with such polarity that all the electropositives such as sodium are moved to the outer surface.