GB529777A - Improvements in or relating to television transmitting and like cathode ray tubes - Google Patents

Abstract

529,777. Cathode-ray tubes. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 24, 1939, No. 15475. Convention date, June 15, 1938. [Class 39 (i)] In a television transmitting or like cathode ray tube the screen comprises a conducting plate having on its image-receiving side a directlydeposited continuous layer of an insulating dielectric material which has been previously reduced to a powdered form. The powder may have a superposed layer of mosaic of the usual discontinuous photo-sensitive elements of the usual form. The screen 7 in the form shown in Figs. 1 and 2 consists of a signal plate 9, of polished metal or of glass coated with evaporated aluminium or chemically-deposited platinum, which carries a layer 11 of China clay sprayed on or deposited from a suspension by settlement or cataphoresis. The screen may be sensitized within the tube by evaporation of silver from tungsten heaters 17, oxidation and subsequent coating with caesium 18. The powder employed should be fine grained, chemically inert, of low vapour pressure and of high resistance, and alternatives to china clay are powdered mica, quartz, aluminium silicate, titanium dioxide and magnesium oxide. Smoking the screen with the products of a burning metal or coating it by evaporating an insulating material are disclaimed. If deposited by cataphoresis, a dispersing agent is used, such as about 0.1 per cent of nitrocellulose in a china clay suspension in acetone or a few drops of ammonia in a water suspension. Binders may be used with some types of powder. There is no critical thickness for the powder layer but as an example a thickness of 2 mils is given for china clay in a tube as shown in Fig. 1. The tube shown in Fig. 4 has, in addition to the electron gun comprising a cathode 22, control electrode 23 and first and second anodes 24, 26, an image tube section comprising a transparent photosensitive cathode 31, focusing electrodes 32, 33 and accelerating electrode 34. The screen consists of a conducting signal plate 28 with a powdered surface 29, as described above. This surface need not be sensitized although, in practice, it may become so by deposit of some photo-sensitive metal when the cathode 31 is sensitized.