Patent GB-447819-A (GB)
Title: Improvements in or relating to television and the like transmitting systems
IPC: H01J31/34
Filed: 1934-09-20
Abstract: 447,819. Cathode-ray tubes. McGEE, J. D., 9, King's Avenue, Ealing, London, and KLATZOW, L., Andora, Somerset Way, Iver, Buckinghamshire. Sept. 20, 1934, Nos. 25203 /35 (divided out of 446,664) and 14428 /35. [Class 39 (i)] In tubes with a mosaic screen which is scanned by a cathode-ray and on which an electrostatic image of an object to be transmitted by television is formed, the potentials of the insulated elements of the mosaic are restored to a datum value by low-speed secondary electrons produced by the impact of the high-speed electrons of the scanning ray on parts shielding the mosaic elements from direct impact of the ray. The mosaic screen 2, is of the double - sided kind scanned on one side by the ray 14 and photo - sensitized on the other side to emit electrons to an electrode 18 when an optical image is focused on to the screen by a lens system 19, the screen 2 including a signal electrode connected to an output resistance 16. The signal electrode is formed by wires 3, Fig. 2, coated with insulation pressed in dies to form a perforated sheet 4 as described in Specification 447,824. The perforations are of depth not less than, and preferably two or three times, their diameter and are closed at the front by silver globules 6 with photo-sensitized surfaces 9. A coating of silver 7 is applied to the back of the sheet 2 by evaporation in an inclined direction 8 so that the silver does not penetrate through the perforations to the globules 6 but merely penetrates to the back end of one side of each perforation as shown at 7<1>. The coating 7 may be insulated or connected to earth and to the anode 12 of the electron gun by a lead 17. The ray 14 arrives at the screen from the same direction as that from which the silver 7 was applied, and strikes the coating 7 and not the globules 6. Secondary electrons emitted by the coating 7 reach the globules 6 to neutralize the positive potentials resulting from the photo-electric emission. The coating 7 may be omitted or replaced by a silver layer aggregated by heat. The globules 6 are formed by dusting on silver or may be replaced by small globules produced by aggregation by heat. The invention may also be applied, Fig. 3 (not shown) to tubes similar to those described in Specification 442,666, in which electrons from a photoelectric electrode are accelerated and focused as by a coil on to the front of a double-sided mosaic electrode scanned on its back by the cathode-ray : the globules 6 on the front of the mosaic are not then photo-sensitized. The invention is not applicable to single-sided mosaics associated with a separate photoelectric electrode on the same side of the mosaic as the scanning ray, but is applicable to single-sided, photo-electric mosaics, Fig. 4 (not shown), comprising a mica sheet between a signal plate and the photo-electric elements scanned by the ray : an earthed metal screen with deep perforations is then placed near the mosaic to intercept the ray and to emit secondary electrons and the optical image is projected on to the mosaic through this screen.