Patent GB-385898-A (GB)
Title: Improvements in and relating to electric discharge devices
IPC: H01J61/64
Filed: 1931-07-28
Abstract: 385,898. Discharge lamps. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Foulke, T. E.; 30, Evelyn Place, Nutley, New Jersey, U.S.A.) July 28, 1931, No. 21563. Convention date, Aug. 2, 1930. [Class 39 (i).] A cathode glowdischarge lamp operating on direct or alternating current, and especially suitable for use with direct current of 100-110 volts, comprises closely spaced electrodes coaxially arranged within the envelope and each coated with an alkali or alkaline earth metal, at least one electrode being of helical form while the other is so constructed as to be capable of being heated to a high temperature by high frequency induction. In one construction, Figs. 3 and 4, one electrode is constituted by a hollow cylinder 5, preferably of nickel or nickel-plated iron, closed at its upper end, and the other by a helix 9, preferably of nickel-plated iron, mounted concentrically around the cylinder. Leads 2, 3, preferably of copper-coated nickel wire, are sealed through the stem press 4, the lead 2 being connected to a transverse supporting member 6, preferably of nickel, within the cylinder 5, and the lead 3 extending through an opening in the top of the cylinder for connection to the helix 9. A tubular vitreous sheath 7 serves to insulate the lead 3 from the cylinder 5 and its upper end provides a further point of support for the cylinder. A downward extension of the support 6 carries a tubular vessel 10, preferably of iron, open at its lower end and enclosing a small quantity of magnesium 11 to serve as a getter. In a modification, Figs. 6 and 7, the leads 21, 31 sealed through the stem press 4 pass through longitudinal passages in a cylinder 12 of lavite, porcelain or other suitable insulating material mounted within the' cylindrical electrode 5<1>. The lead 2<1> is spot welded to a tab 14 punched up from the top of the cylinder 5<1> while the lead 31 passes freely through an opening in the top of the cylinder 5<1> for connection to the helix 9. The getter vessel 10 is carried by a wire 13 which passes through a third longitudinal passage in the cylinder 12, its'upper end being bent over and held between the tops of the cylinders 12 and 5<1>. In each case the electrode structure is first assembled with the exception of the electrode 9. The electrode 5 or 51 is then coated with an alkali or alkaline earth metal compound decomposable to the oxide, e.g. barium or strontium carbonate or potassium hydroxide. The electrode 9 is then mounted, the stem 4 is sealed into the envelope, and the lamp is heated to 450‹ C. for 10 mins. while maintaining a high vacuum for the removal of occluded gases. A high frequency coil is then held for 15 secs. with its axis normal to that of the electrode structure to raise the temperature of the electrode 9 to about 600‹ C. to remove from its surface any oxide. The coil is then placed with its axis in line with that of the electrode structure to raise the temperature of the electrode 5 or 5<1> to about 1000‹ C. for a time sufficient to effect the decomposition of the carbonate or hydroxide to the oxide. The coil is then lowered to concentrate. its heating effect on the magnesium 11 to vaporize it. The envelope is then filled with mercury vapour, neon, helium or other suitable gas or vapour, or mixtures thereof, e.g. neon containing 0.2 to 0.8 per cent. of argon at 35 mm. pressure. After sealing off, the lamp is connected to a source of pulsating or alternating potential of steep wave front to produce a bombardment of the electrode 5 or 5<1> serving to reduce some of the oxide to the free metal and to sputter some of the metal on to the electrode 9. To obtain a more uniform coating on this electrode it is preferred to use an unsymmetrical alternating current such that the half cycle of greater potential occurs when the electrode is negative. Fig. 8 shown a modified construction in which two helical electrodes 24, 25 are employed, the turns of each helix being shortcircuited. The helix 24 is connected at each end to the lead 22 and the helix 25 at each end to the lead 23 and the turns of one helix are evenly spaced between those of the other. Both electrodes are coated with the alkali or alkaline earth compound which is subsequently decomposed to the oxide by the heat generated in the closed circuits of the helices by a high frequency field and thereafter reduced to the free metal by the action of an alternating potential between the electrodes. Specifications 162,268, [Class 39 (i), Electric lamps, Arc &c.], and 362,346 are referred to.