Patent GB-209833-A (GB)
Title: Improvements in vacuum tubes and the like
IPC: H01J5/06
Filed: 1922-10-18
Abstract: 209,833. Robinson, E. Y. Oct. 18, 1922. Condensers.-In vacuum condensers with imperfect vacuum, undesired gaseous discharges, e.g. between the leading-in wires, are prevented by dividing the gap across which the discharge would normally occur into a number of gaps in series. The invention is shown in Fig. 1 applied to a thermionic valve in which the distance between the anode 1 and the cathode 2 is sufficiently small to prevent glow discharge. The anode 1 consists of a closed copper tube sealed to a vitreous tube 6 at 5, to which tube is also sealed a re-entrant tube 7. The cathode 2 consists of a tungsten tube heated by thermionic bombardment or otherwise and supported by stems 3 from a metal tube 9 which grips the re-entrant glass tube 7. A long leakage path is therefore provided between the electrodes 1, 2, and the electric stress across this gap is confined between a shield 11, connected to the anode by a conical portion 12, and a shield 10 connected to the cathode. The gap between these two shields is divided into a number of smaller gaps in series by means of a plurality of shields 13, which may be held by three glass rods 14 and glass spacing-tubes 15. The shields 10, 11, 13 may be flanged or beaded as shown to trap the electrons and prevent local stresses in the glass. Shields or guard rings 18, 19 may be arranged outside the tube 6 and connected to the end shields 10, 11 to make the electric stress outside the tube correspond with that inside the tube and to protect the seals from dielectric strain. The seal 5 is shielded by the conical tube 12, and the cathode-supporting stems 3 are shielded by a tube 4. In a modification, the shields 13 are formed by concentric flanged metal tubes spaced by glass tubes between the flanges. The shields 13 may be uniformly graded as to potential by connection to tappings on a high resistance connected between the two electrodes, for example a deposit of carbon, gold, tungsten, molybdenum, nickel, &c. on the walls of the tube or on a glass or silica support. Where alternating currents are employed, tappings may be made to an inductance or an oscillatory transformer, or the electrical capacity of the shields to each other and to each electrode may be varied by suitably proportioning the guard rings 18, 19 and the shields 13 or by capacitative coupling to a series of guard rings mounted on the outside of the tube. The tube may be provided with petticoats on its external surface to increase the external leakage path, the space between the petticoats and the tube being filled with oil. Fig. 2 shows a modification enabling continuous evacuation to be carried out by connecting the lower end of the anode to a mercury-vapour or other vacuum pump. The cathode 2 is provided with a shield 24 for reducing ionization as described in Specification 195,691, [Class 39 (i), Ecletric lamps, Arc &c.]. An annulus 22 and disc 21 are attached by stems 23, forming a shield which prevents gaseous discharge. The shield is in contact with the anode 1 and is supported by a stem 25 from a ring 26 screwed into an extension of the anode and held by a locking-ring 27. Alternatively, the shield at the end of the anode may be formed as a grid, or of a perforated sheet of metal. Two of more of such shields may be employed having the perforations staggered.