GB857711A - Improvements in or relating to discharge lamp envelopes - Google Patents

Abstract

857,711. Discharge lamps. GENERAL ELECTRIC CO. April 3, 1957 [April 19, 1956], No. 10932/57. Addition to 821,321. Class 39(1). A discharge lamp, particularly a fluorescent lamp, of the parent Specification having a vitreous envelope of generally tubular form with a transversely re-entrant groove extending longitudinally of the envelope to form a discharge space 1 with a kidney-shaped crosssection, has the regions of maximum stress at the bottom C of the concave groove forming wall portion 18 and at the convexly curved wall portions 19 and 19<SP>1</SP> on either side of the groove of increased wall thickness relative to the remainder of the envelope wall. The ratio of maximum to minimum wall thickness throughout the cross-section of the envelope is not in excess of approximately 3 : 1 and is preferably approximately 2 : 1. The side walls 21 and 21<SP>1</SP> of the groove are substantially straight portions inclined at an angle of more than 15 degrees, e.g. 27 degrees, to the medial plane through the groove. Concave inner wall 18 has a greater curvature than convex outer wall 17 and which can be three times as great, and convex wall portions 19 and 19<SP>1</SP> have a slightly greater inside curvature than concave wall 18. The filling comprises an inert gas, e.g. argon at a few mms. pressure, and mercury vapour at a pressure of 1 to 20 microns. The envelope can be made by drawing grooved and differentially thickened tubing directly or by reforming a differentially thickened tube of circular section. The reforming can be done by heating the round tube to its softening point and then clamping a suitable mould about it, or by heating the tube in a selected zone corresponding to the region of the groove to its softening temperature, e.g. approximately 540‹C. in the case of lime glass, and pressing on the tube in the heated region a plunger with a protuberance.