Patent GB-191415788-A (GB)
Title: Improvements in and relating to Electrical Vacuum Discharge Devices.
IPC: H01J21/02
Filed: 1914-07-01
Granted: 1915-10-01
Abstract: 15,788. British Thomson-Houston Co., [General Electric Co.]. July 1. Vacuum tubes.-A tube containing a hot cathode, an anode, and a grid or other conductor modifying the discharge, is exhausted to such an extent that there are no indications of gas ionization, such as a blue glow at the cathode. A negative charge on the grid decreases, and a positive charge increases, the current between the electrodes. Special effects are obtained when a certain gas or vapour pressure, too low for ionization, is maintained. The cathode may be a tungsten, tantalum, or other filament, in the form of a single straight length, one or more V-shaped pieces connected in parallel, or a long piece wound to and fro on two parallel tungsten rods. One or more springs maintain the cathode filament taut within a closed supporting-frame 9', Fig. 1, of glass, silica, tungsten, ferro-chrome, or other material. Glass buttons may in some cases be placed on the cathode conductors to prevent short-circuiting. Over the frame 9' may be wound a tungsten or other wire forming the grid. If the frame is of metal, the cathode conductors are insulated by glass supports, and mica sheets separate the frame and anode. In apparatus suitable for converting direct into alternating currents, additional similar grids may be placed between the first grid and anodes, the first grid being positively and the others negatively charged. Instead of a grid, there may be used a flat ring 57, Fig. 8, placed with its aperture just in front of the cathode 58, which is partly surrounded by a cup 62, electrically connected to the cathode so as to direct the rays. This arrangement may be used for rectifying current. Anodes may consist either of tungsten or like filaments 12, Fig. 1, which may be loosely strung on hooks on one or more forked frames, or of disks mounted on tungsten or molybdenum stems. Exhaustion may be effected by a Gaede pump; chemical absorbents such as electrically vaporized calcium or magnesium may also be used. During exhaustion, the tube is strongly heated, and the anode may be brought to incandescence by passage of current. Evolution of gas from the anodes is promoted by cathode-ray bombardment, which may also effect the heating when the anodes are of disk form. An increasing electric pressure is applied, the final pressure being preferably higher than the working pressure unless this is very large, say 50,000 volts. Anodes so treated may be removed from the tube without re-absorbing gas. The residual gas pressure may be a few hundredths of a micron; it must be lower when the electrodes are widely separated than when they are close together. If a definite gas or vapour pressure, too low for gas conduction, is maintained in the tube, and an electric pressure within a particular range is applied between the grid and cathode, the superposition of electric oscillations on the grid causes an immediate fall of conductivity of the tube. A suitable electric pressure is 1À2 volts. The vapour may be that of sulphur, phosphorus pentoxide, or tungsten hexachloride. Mercury may be used if placed in a cool side chamber or converted into an amalgam. For this purpose, silver may be placed in the tube, and mercury in a pocket which is kept cool until after exhaustion, when it is slightly warmed. After one or two days, the silver is sufficiently amalgamated by the vapour, and excess of mercury is then removed from the tube by heating the tube while strongly cooling the pocket, which is subsequently sealed off. Specifications 24,850/04, [Class 38, Electricity, Regulating &c.], 1427/08, and 14,892/13 are referred to.