GB646369A - Improvements in and relating to electric discharge lamps - Google Patents

Abstract

646,369. Discharge lamps. BRITISH THOMSON-HOUSTON CO., Ltd. July 9, 1946, No. 20499. Convention date, April 21, 1945. [Class 39 (i)] In a long gap low-pressure positive column discharge lamp with a coating of luminescent material on the interior wall of the envelope, and having a filling of starting gas and vaporizable metal, both of the thermionic electrodes 8 are surrounded by open-work metal screens 11 which have a floating potential during operation. As shown, the stem 2 flares at its inner end 3 so as to elastically distort the woven wire mesh screen 11 when it is slipped over the stem; the screen is thus held securely in place. A small lug 14 may key the screen in place, or the screen may be sealed to the stem. An exhaust tube 6 is provided, and the filling may be argon at 2-6 mms. together with mercury vapour at 10 microns. The thermionic electrodes 8 may be operated on A.C. or D.C. and may be used for hot or cold starting, and may be constructed as described in Specification 569,685, [Group XX]. The screens 11 may be of coarse or fine mesh and may be of uniform or non-uniform mesh. The front end 12 of the screens may be omitted, Fig. 3 (not shown). The screens are stated to reduce " spiralling " of the discharge, " anode oscillations " and end blackening." The Specification as open to inspection under Sect. 91 states that only one electrode need be provided with a screen 11, which may be of perforated sheet-metal or a closely-wound helix of fine wire. The filling may be of gas alone or of metal vapour alone. For D.C. operation only one of the electrodes may be thermionically emissive. Parts of the screen 11, such as portion 12, Fig. 2, may be imperforate, and parts such as the rear end or both the front end 12 and the rear end, may be left open. An imperforate disc or tube is, however, disadvantageous. The screen 11 should lie well within the Faraday dark space. In another embodiment, Fig. 4 (Cancelled), each electrode 8 is surrounded by two concentric tubular wire mesh screens 11, 16. The screens 16 are connected inside or outside the envelope 1 by a connection 15 of negligible resistance which is preferably broken after starting. If the outer screens 16 are omitted and the screens 11 are connected by connection 15, a resistance of ¢ megohm must be inserted in the circuit to prevent shorting out of the main discharge. This subject-matter does not appear in the Specification as accepted.