Patent GB-863643-A (GB)
Title: Improvements in or relating to sodium vapour electric discharge lamps
Inventor: RIGDEN SYDNEY ALFRED RICHARD
IPC: H01J61/34
Filed: 1958-04-22
Abstract: 863,643. Discharge lamps. GENERAL ELECTRIC CO. Ltd. April 10, 1959 [April 22, 1958], No. 12722/58. Class 39(1). A discharge lamp comprises an envelope tube 1, of circular or oval cross-section, filled with a starting rare gas and with sodium vapour at low pressure-less than 1 mm. and about 0.01 mm., the envelope being mounted, or arranged to be mounted, within at single or double walled evacuated outer jacket 3 and having, over at least the greater -part of its overall length, substantially the form of a helically coiled tube including at least two complete turns, such overall length measured in a direction parallel to the helical axis being greater than the overall width of the envelope. The helically Coiled part of the tube 1 can be in the form of a single-start helix having a pitch which is not more than twice, and is preferably approximately one and a half times, the width of the envelope tube cross-section. Alternatively the helically coiled part of the tube can be in the form of a two-start helix, with the turns of the two helical elements evenly interleaved and connected together at one end of the helix, and with the pitch of each helical -element not more than four times, and preferably approximately three times, the width of the envelope tube cross-section. The diameter of the helix into which the envelope tube is coiled can be less than one half of the axial length of the helix. The discharge envelope can be sealed within an evacuated singlewalled outer jacket or mounted within a double-walled outer jacket, either of the kind in which. the envelope is sealed within the outer jacket with the space between the envelope and the inner wall of the jacket evacuated, or of the kind. in which the envelope is detachably mounted within an evacuated outer jacket with air present in the space between the envelope and the inner wall of the jacket. In the construction shown in the drawings the ends of the envelope 1 containing the electrodes 2 extend outwards from each other parallel to the axis of the helix and project through closely fitting holes in a pair of flexible discs 11, 12, e.g. of mica, which fit closely against the inner surface of the outer jacket 3, Fig. 1, or of a hollow cylindrical sleeve 19, Fig. 2, of transparent heat-insulating material, such as glass, interposed between the envelope 1 and the jacket 3. If desired the envelope at one end of the helix can be extended back along the middle of the helix so that the ends of the envelope are located at the same end of the helix. With 'a two-start helical envelope the ends of the envelope can also be located at the same end. The disc 11, Fig. 1, is held in position by a support wire 18 welded to one of the lead-in wires 6. The protruding ends of the electrode leads 7 at the foot-tube end of the envelope are bent radially outwards and connected to the turned-out end of the lead-in wire 6, by a welded metal sleeve 13. At the other end of the.envelope the electrode leads 7 are connected by a metal sleeve 15 to a wire 14 extending along the middle of the helix within a glass capillary tube, the opposite end of the wire being connected to the other lead-in wire 6. The evacuated space within the jacket 3 contains a getter element which may be spotwelded to one of the lead-in wires 6 and provides a coating of getter material on the internal wall of the jacket near the foot-tube 5. In the construction shown in Fig. 2 the cylindrical glass sleeve 19 is held in position by additional mica discs 20, 21, the disc 20 being supported by the lead-in wire 6 which urges the disc 20 against the end of the sleeve 19 and thus the Other end of the sleeve against the other disc 21. Transverse movement of the sleeve 19 is prevented at each end by a pair of wires 22 of approximately U-shape extending into the sleeve through holes in the discs. The ends of the wires 22 also bear against the discs 11 and 12. A getter element is provided within the evacuated jacket 3. The envelope can be constructed as a unit for detachable insertion within a double-walled Dewar jacket and can incorporate an insulating support member rigidly connected to one end of the envelope, the support member carrying input terminals and being shaped to fit within the central hole in an annular base member sealed to the open mouth of the Dewar jacket. The filling can be neon containing 0.8% by volume of argon at a total pressure of about 10 mms. together with about 2 grs. of sodium. The envelope is coated internally with a layer of sodium resistant glass.