GB913963A - Improvements in or relating to symbol displaying electron discharge tubes - Google Patents

Abstract

913,963. Cathode-ray tubes. ENGLISH ELECTRIC VALVE CO. Ltd. Feb. 2, 1961 [May 16, 1960], No. 17129/60. Class 39 (1). A symbol displaying electron discharge tube comprises, within an evacuated housing 1, a fluorescent screen 4, an electron gun 2 arranged to flood the screen uniformly with electrons, and two spaced mesh electrodes 6, 7, extending transversely across the beam having between them a plurality of symbol electrodes 9 each provided on the side facing the gun with a layer of insulating material having a high co-efficient of secondary emission and a low first secondary emission cross-over. In operation, a negative pulse applied to one of the symbol electrodes lowers its potential substantially below the first secondary emission cross-over potential of the insulating material at which it is initially held by the mesh electrodes, and is held at cathode potential by the electron beam, thus casting a shadow on the fluorescent screen. Erasure is by application of a negative pulse to the cathode, the velocity of the electron beam being such as to charge-up the symbol electrodes to the potential of the nearest secondary electron collector, i.e. the mesh electrodes. Suitable insulating materials are magnesium fluoride, magnesium oxide and talc. The electron beam from the gun 2 is accelerated by electrode 11 and collimated by electrode 12 so that the electrons travel in parallel paths to the first mesh electrode 6.