Patent GB-704848-A (GB)
Title: Improvements in and relating to high pressure electric discharge devices
IPC: C03B23/047
Filed: 1949-10-24
Abstract: 704,848. Discharge lamps. BRITISH THOMSON-HOUSTON CO., Ltd. Oct. 24, 1949 [Oct. 29, 1948], No. 27271/49. Class 39(1) [Also in Group XXIII] A high pressure gas and/or vapour filled discharge device, preferably a lamp, has a transparent vitreous quartz envelope portion coated externally with a transparent glaze to prevent penetration of hydrogen from the atmosphere and consequent devitrification, and the glaze is free from compounds of alkali or alkaline earth metals It is stated that the latter compounds are excluded because they are unsuitable. The pressure may be ¢-100 atmospheres or higher The inside of the envelope portion may also be coated with glaze for the same purpose. It is explained that the hydrogen is formed by photo-breakdown of water vapour when subjected to the ultra-violel radiation of the are discharge. The glaze may consist of B 2 O 3  and/or P 2 O 5  which has a lower fusing point than quartz and a stabilizer such as Al 2 O 3  which has a higher fusing point than quartz, or silica powder, may or may not be added. Relatively high melting point glazes such as alumina, thoria, zirconia, beryllia or titanium dioxide may be used alone. The union of the glaze materials with one another and with the envelope may be physical e.g. a solid solution, or chemical e.g. silicates may be formed. The starting materials may be chemical combinations of components e.g. B 2 O 3 ÀP 2 O 5  or SiO 2 ÀP 2 O 5 . Phosphates may be used in conjunction with borates, and may not break down to oxides, or may be used separately. B 2 O 3  can be introduced in the anhydrous form, as H 3 BO 3  or BN (which reacts with oxygen of the air) or as a borate. P 2 O 5  can be introduced as such, as H 3 PO 4  or in phosphates such as (NH 4 ) 2  HPO 4 . Silica may be introduced as such or as H 3 SiO 3 , and titanium oxide as the nitride. It is stated that it is doubtful whether the nitrides are converted to oxides. Specific examples of glazes are given: (a) 31.2 gm. H 3 BO 3 , 15.6 gm. silica, 235 cc. acetone, (b) 15.6 gm. silica, 34.4 gm. (NH 4 ) 2 HPO 4 , 235 cc. acetone, 15 cc. 85 per cent H 3 PO 4 , (c) 83.5 per cent silica, 4.5 per cent Al 2 O 3 , 12 per cent B 2 O 3 . After coating the glaze is fired by heating with an oxy-hydrogen flame. In an alternative construction a hollow quartz slug is glazed and then drawn out into tubing, the glaze stretching with the quartz, and the tubing is cut up into lengths for envelopes. The quartz envelope may have end caps of borosilicate glass carrying the electrode leads as described in Specification 27133/45 (as open to inspection under Sect. 91). The glaze may overlap the fused joints between the quartz envelope and the glass end caps. The glaze may be applied by dipping the envelope, held horizontally, into a suspension of B 2 O 3  in butyl acetate and rotating the envelope. The lamp may have a glass jacket as disclosed in Specification 675,201, filled with nitrogen at ¢ atmosphere. In this form, Fig. 3, the tube is carried in the jacket 14 by U- shaped wire mount 17 with bridging members 18, resilient braces 20 and heat shield 22; the mount 17 is connected directly to the upper main electrode, and to the starting electrode through resistance 21, while the other main electrode has a separate lead. The filling may be one or more metals such as Hg, Cd, Zn, Tl and/or one or more rare gases such as A, Xe at a few mm. Specifications 441,603 and 599,489 also are referred to.