Patent GB-267141-A (GB)
Title: Improvements in or relating to electric discharge devices
IPC: H01J19/44
Filed: 1927-03-04
Abstract: 267,141. British Thomson-Houston Co., Ltd., (Assignees of Kingdon, K. H.). March 6, 1926, [Convention date]. Low-pressure metallic-vapour apparatus; vacuum tubes.-In electric discharge devices containing a conducting material, such as alkali metal, of low vapour-pressure and tending to deposit on the interior surface during manufacture or operation of the device, a continuous leakage-producing deposit is prevented from forming on the stem or other part of the interior surface by shielding the stem or other part from the effects of ionization and by producing, by allowing water vapour to remain in the device and in the walls thereof, a condition of the interior surface such that any deposit formed is a discontinuous deposit of isolated particles. Traces of oxide on the stem tending to produce a continuous deposit by absorbing vapour are converted into hydroxide by the water vapour, whilst gas ionization adjacent the stem tending to promote the formation of a film of such absorbing traces is prevented by shielding the stem from electrons emitted by the cathode. The straight cathode 4, Fig. 2, comprising a spiral of fine tungsten wire connected to nickel wires on heavier supporting wires 5, 7 may be shielded from the stem 2 by an annular plate 14 adjacent to, but out of contact with. the lower end of the cylindrical anode 8. The plate 14 carries a control electrode comprising radiallv bent plates 9, 13 with central apertures and the anode carries internal radial vanes 10. A capsule 17 comprising two metal plates, one with a cavity, is supported by a wire 18 from the anode 8 so that the capsule 17 can be heated independently of the electrodes and is filled with a mixture yielding an alkali metal, for example, a mixture of caesium chloride and calcium or magnesium to yield caesium. During exhaustion, the bulb is partially dehydrated by heating to below 300‹ C. for 30 minutes and the electrodes are freed from gas by heating by high-frequency induction or otherwise. Phosphorus pentoxide in a side bulb is used instead of a liquid-air trap to allow some moisture to remain on the glass. The tungsten filament is provided with an oxygenous surface, suitable for the absorption of caesium giving a high electron emission as described in Specification 208,729, by introducing oxygen at 100 microns pressure and heating the filament to 2000‹ absolute. After exhausting the oxygen, the capsule 17 is heated to liberate caesium. A source of oxygen such as a piece 19 of oxidized copper may be provided, the caesium oxide produced, together with the metallic caesium, eliminating hydrogen, carbon monoxide and other gases evolved from metal or glass parts. The stem, Fig. 3, may be provided with a shield 20 partly enclosing it, the electrodes being otherwise unshielded. The grid may comprise slotted plates 22 connected together at their upper ends by a wire and inserted through slots in plates 21 forming the anode. One end of the cathode 4 is connected to and supported by the shield 20. The devices may be used as amplifiers with the grid connected to the negative end of the filament.