GB542219A - Improvements in cathode ray tubes, particularly for television transmission - Google Patents

Abstract

542,219. Cathode ray tubes. MARCONI WIRELESS TELEGRAPH CO., Ltd. May 27, 1940, No. 9295. Convention date, May 27, 1939. [Class 39 (i)] In a television transmission cathode-ray tube the screen such as the photo-sensitive mosaic electrode 2 is scanned by low velocity electrons from a gun 3 the line deflection being effected by electrostatic plates 16, 17 wholly within a magnetic field produced by the electromagnet 18 ; to obtain uniform resolution of the picture over the whole of the mosaic the plates 16, 17 are formed to give a field which first increases substantially uniformly in intensity then remains uniform and finally decreases uniformly. By this means the transverse velocity of the electrons which produces the spiral path along the magnetic field is restricted to the initial velocity at emission and the helix remains of small diameter. The curved parts of the plate are preferably hyperbolic and their lengths are related to the helices formed by the electrons ; these parts may be separated by straight portions. In a modification the electrons pass between pairs of plates 21, 22 and 23, 24, Fig. 4a at the average potential of the deflecting plates 16, 17; the edges of these pairs are separated a distance a equal to an integral number of helices and in consequence by adjustment of the tube parameters the electrons may be made to enter and leave the deflecting field without any transverse velocity component additional to that obtained at emission. To prevent fringing the plates in each pair should be closed. To restrict the electro-static field of the deflecting plates a slotted screen 25 is placed between the target and plates 16, 17; an electrode 8 catches electrons not formed into the beam by the gun and a wall coating 27 is also provided. The frame deflection is effected by the electromagnetic coils 19, 20. U.S.A. Specifications 2,020,305 and 2,065,570 are referred to.