Patent GB-866198-A (GB)
Title: Improvements in arc tube seal and mount
IPC: H01J9/32
Filed: 1957-08-09
Abstract: 866,198. Discharge lamps. GENERAL ELECTRIC CO. Aug. 9, 1957 [Aug. 30, 1956(4)], No. 25189/57. Class 39(1). [Also in Group XXXVI] A vitreous electric arc tube having at each end an axially projecting flat pinch seal and support wires embedded in the pinch seals for the support of the arc tube in an outer jacket for example, has each pinch seal of a width at least substantially as large as the diameter of the tube and with a lead-in conductor hermetically sealed through its central portion, and has the support wires extending in an axial direction outwardly from the seal near its lateral edges. The high pressure mercury vapour-inert gas lamp 1 shown in Fig. 1 comprises a vitreous bulbous jacket 3 enclosing an inner quartz arc tube 12 containing main electrodes 13, 14. Each electrode comprises a core 15, which is the inward projection of its lead-in, made of tungsten or molybdenum and surrounded by a tungsten wire helix 16. A small elongated piece of thorium metal 17 is positioned between the core and helix to reduce the cathode drop at the electrodes during operation. A starting electrode 18 consisting of the inward projection of a lead-in is positioned adjacent the electrode 13. The lead-ins for electrodes 13, 14 and 18 each comprise a molybdenum foil section 19 bonded to the pinch seals 21, 22. The arc tube 12 has a filling of mercury, at a pressure of the order of one-half to several atmospheres during operation, and inert gas, e.g. argon, at a pressure of about 25 mms. The pinched seals 21, 22 extend to at least the diameter of the tube 12. They can be made by flattening the circular ends of the tube without prior necking and are then of a width approximately #/2 times the diameter of the tube. The tube 12 is supported in the jacket 3 by support wires 23, 24 in the corners of the seal 21 and support wires 25, 26 in the corners of the seal 22. Conical-shaped cavities 27 are provided where the support wires penetrate into the seals. The inner ends of the support wires are flattened at 28. The support wires 23, 26 are provided with bends within the seals which are welded to the outer ends of the foil sections 19 of the electrode lead-ins. Support wires 23, 24 are welded to the laterally turned ends of the lead wires 6, 7. Starting electrode 18 has a wire portion 29 welded to the outer end of its foil section 19 and a current limiting resistor 31 is connected between the wire 29 and lead wire 7. Support wires 25, 26 are fastened to a transverse brace member 32 having spring fingers 33 bearing against the jacket 3. A flexible wire 34 connects support wire 26 to the end of lead wire 7. A heat reflector disc 35 is mounted above the ends of lead wires 6, 7, is welded to lead wire 6 and is slotted at 36 to avoid contact with the lead 7. In the high pressure mercury vapour arc tube shown in Fig. 3 the wire portion 29 of the lead-in for the starting electrode 18 is eliminated and a current limiting resistor 31a is provided having leads of substantially the same thickness as the support wires, at least the upper lead 37 being of refractory metal such as molybdenum. The lead 37 enters the seal 21 through the conical aperture 27 and has a right angle bend welded to foil section 19 of the lead-in for starting electrode 19. The other lead 38 of resistor 31a is welded to the turned end of the lead wire 7, the resistor 31a forming a support for the tube 12. Fig. 4 shows apparatus for forming the pinch seal 21 in which the support 23 is not connected to a lead-in foil 19. The support 23, lead-in 13<SP>1</SP> for main electrode 13, lead-in 29 for starting electrode 18 and support 24 have their lower ends supported in apertures in the upper surface of a cylindrical pin 41. The quartz tube 12 is supported either by an exhaust tubulation projecting laterally or by a chuck engaging its body and is held so that its lower surface just clears the top surface of the pin 41 as shown at 12<SP>1</SP>. After heating the lower end of the tube 12 to plasticity by a pair of burners the jaws 43, 44 having concave upper surfaces 45 are pressed together. The seals for the constructions shown in Figs. 1 and 3 can be similarly formed, the lead-in foils and electrodes being previously welded to the bent upper ends of the supports. The jaws can have shallow grooves 39<SP>1</SP> and 40<SP>1</SP> in transverse and axial directions to restrict the flow of plastic quartz on pinching and to cause the formation of ridges 39 and 40 in the seal surface. The jaws are also provided with cavities 46 at their lower edges to form the conical apertures 27. The jaw surfaces are shaped so that the seal is tapered with a thickness decreasing from the axial line to the lateral edges, providing a thickness differential in the range of 0.7 to 2 mms. An axial increase in the thickness from the curved inner end to the outer end of the seal of 0.1 to 0.5 mm. can also be provided. Tables are given showing for different jaw widths the inclinations of the jaw surfaces for producing different transverse thickness differentials. The axial thickness differential can be obtained by tipping the jaws 43, 44, relatively to their operating levers.