Patent GB-668727-A (GB)
Title: Improvements relating to electron discharge devices
Inventor: BROADWAY LEONARD FRANCIS
IPC: H01J21/02
Filed: 1948-07-07
Abstract: 668,727. Cathode-ray tubes. ELECTRIC & MUSICAL INDUSTRIES, Ltd. June 22,1949 [July 7, 1948], No. 18203/48. Class 39 (i). A method of amplifying an electron current consists in accelerating electrons on to a layer of material with such a velocity that they penetrate the layer and cause induced conductivity and cause a current to flow through the layer which is greater than the incident electron current. Suitable substances for the layer are photo-conductive substances such as selenium, zinc sulphide, and copper oxide and also semi-conductors such as silicon and germanium and insulators such as magnesium oxide and fluoride, titanium oxide, and silica. The thickness of the layer is such that the high velocity electrons penetrate the layer as completely as possible to avoid polarization effects. The layer should preferably be coherent as distinct from porous. Fig. 1 shows a television pick-up tube comprising an evacuated envelope 1 containing a target electrode which comprises a conducting grid 2 and a thin aluminium sheet 3 which supports a continuous layer 4 of a material exhibiting induced conductivity under electron bombardment. At one end of the envelope 1 is a photo-electric cathode 5 in contact with a transparent conducting electrode 6 and receiving an optical image from a lens system. Photo - electrons are released and accelerated towards the target by an electrode 7 and are focused by a coil 8. The opposite surface of the layer 4 is scanned by an electron beam which is generated by a thermionic cathode 9 surrounded by a screen 10 and is accelerated by an electrode 11 and wall anode 12, the scanning being effected by coils 13. The elemental areas of the surface of the layer 4 are thereby restored to an equilibrium potential and generate picture signals across an impedance 14 connected to the conducting sheet 3. Current much greater than the photo-electric current flows through the layer 4 so that a substantial degree of amplification is obtained. In a modification the photo-electric image is projected on to the surface of the layer 4 that is scanned by the electron beam, the electron gun in this case being arranged in a portion of the envelope 1 oblique to the target electrode. The target electrode may alternatively be scanned by a low-velocity beam so that the equilibrium potential corresponds to that of the cathode. Fig. 4 shows a tube for use in radar, in which the target electrode is adapted to store electrical signals which are applied to the surface of the layer 4 from an electron gun comprising a cathode 20, modulating electrode 21, and accelerating electrode 22. Signals to be applied to the layer 4 are fed to the modulating electrode 21 so as to vary the intensity of the beam from the cathode 20 and the modulated beam is then scanned over the surface of the layer 4 by scanning coils 23 and accelerated to a sufficiently high potential by a wall anode 24 to cause the layer 4 to exhibit induced conductivity. The opposite side of the target electrode is scanned a number of times by an electron beam generated by a cathode 25. Fig. 5 shows a thermionic amplifier tube comprising an envelope 1 containing a thermionic cathode 30, a control electrode 31, and a target electrode comprising a thin conducting sheet 32 between which and a conductive electrode 33 is arranged a layer of material 34 which can exhibit induced conductivity. The control electrode 31 is connected to a point of negative potential on a potentiometer 35, the cathode being earthed. The potential of the control electrode 31 is modulated by a signal source 37. The conducting sheet 32 is connected to a point on the potentiometer such that there is a sufficiently large potential difference between the sheet 32 and the cathode 30 to accelerate the electrons from the cathode 30 to such an extent that they penetrate the sheet 32 and layer 34 so as to cause this layer to exhibit induced conductivity. The electrode 33 is connected to a point of higher potential than that of the sheet 32 through a signal resistance 38. Instead of a thermionic cathode a photo-electric cathode may be used as in photo-electric cells employed in connection with the sound track of a cinematograph film. The invention may be applied to electronic calculators and for television, where signals are transposed from one standard to another. It may also be applied to image dissectors in which an electron image is scanned over a target electrode.