Patent GB-195691-A (GB)
Title: Improvements in vacuum tubes
IPC: H01J21/02
Filed: 1921-12-06
Abstract: 195,691. Robinson, E. Y. Dec. 6, 1921. Vacuum tubes; low-pressure metallic-vapour apparatus.-A thermionic valve or other vacuum tube has a cathode of the type providing a concentrated source of electrons, with an anode in close proximity to the cathode so that the interelectrode space is small, and means, such as electrostatic or mechanical shields, for confining the space current to the inter-electrode space. By this arrangement a valve having hard characteristics is obtained with comparatively high pressures of residual gas or vapour. In an example the gap between the anode and cathode is stated to be 0.5 mm. with mercury vapour at a pressure of 1 to 1.5 microns, the tube being directly connected to a mercury vapour pump; when the tube contains a gas such as helium the gap may be longer but is always made as small as possible. In one arrangement, Fig. 7, a cylindrical cathode i is heated by a discharge from an auxiliary filament cathode j and is surrounded by a grid 5 and a cylindrical anode k, or the grid may be omitted. The auxiliary cathode is mounted on supports p, r and is maintained at a negative potential with regard to the main cathode. The auxiliary and main space currents are confined to the inter-electrode spaces by shields m, n, mounted on the support p, and maintained thereby at a negative potential; or the shields may be insulated by mounting on quartz beads attached to extensions of the support, and may be left insulated or connected with a source of potential. The cathode may be heated by radiation from the filament without an auxiliary discharge, in which case the surface of the shields must be increased to prevent electron emission therefrom. The shields may be formed by winding tungsten wire to the desired shape. preferably with the turns touching each other, or tubular shields may be used. The grid consists of parallel wires 5 supported on rings 6, 7 and held taut by springs 8. The ring 6 is slotted to receive the wire. The rings act as auxiliary screens, and are supported on insulating beads 10 'so that the gap between the screens and the anode is as small as possible. In another arrangement, Fig. 4, the main cathode consists of two plates x enclosing an auxiliary filament cathode t, the adjacent lengths of which are arranged as close together as possible, so as to provide a concentrated source of electrons without appreciable ionization. A shield v is mounted around, and insulated by a small gap from, the edges of the plates x, and is connected with the negative end of the filament. The anode consists of two metal 'sheets spaced a short distance from the cathode. In the, arrangement shown in Fig. 5, the cathode consists of a closely wound tungsten helix i of circular or rectangular wire, and is mounted in a cylindrical anode k. Shields m, n are mounted at the ends of the cathode. A filament of rectangular section may be formed by winding a helix of circular wire on a mandrel, heating to a 'temperature above red heat in a reducing atmosphere, and swaging or rolling. The helix is then pulled out to space the turns and heated to set the wire. In another arrangement a filament cathode is wound in a zig-zag path parallel with the axis of the electrodes, and supported by spring spiders, the shields being of cylindrical shape with one end closed. The shielding may be effected by maintaining the edges or other portions of a cathode at a low temperature so as to prevent the emission of electrons, for instance., by reducing the heating current supplied to those portions, or by making them of large surface, or of a material with low electron emission. A tubular cathode of tungsten may be coated with thorium, and may be heated to a temperature at which the emission from tungsten is negligible, the tube: thereby acting as an electrostatic screen confining the space current from the thorium.