Patent GB-474845-A (GB)
Title: Improved method and apparatus for measuring low gas pressures
IPC: H01J41/06
Filed: 1936-06-29
Abstract: 474,845. Determining physical qualities of materials. MEYER, L. W. (Naamlooze Vennootschap Philips' Gloeilampenfabrieken) June 29, 1936, No. 18033. [Class 37] [Also in Group XL] A method of measuring gas pressures lower than 0À01 mm. of mercury consists in producing a magnetic field in the gas, establishing a glow discharge in this magnetic field, and determining the gas pressure from the strength of the discharge current, the configuration and strength of the magnetic field being such that the ignition voltage is smaller and the discharge current higher than in the absence of the magnetic field. The apparatus shown comprises a cylindrical glass vessel 1 containing two rectangular parallel nickel or other plates 2, 3 connected by a strap 4 and supported by a lead-in wire 5. Between the plates 2, 3 is a rectangular wire frame 7, of greater length and width than the plates, supported by a leading-in wire 8. Outside the vessel 1 is a C-shaped permanent magnet 10, the flux of which passes from one of the plates 2, 3 to the other, the frame 7 enclosing but being situated outside this flux. The frame 7 may be replaced by a cylinder surrounding the magnetic field between the cathode plates 2, 3 ; this cylinder may completely enclose the space between the plates. In series with the lead-in wires 5, 8 are a high resistance 11, a direct current supply 12, and a glow discharge tube 14 arranged so that the extent of the glow along the cathode 13 is a measure of the current. The glow discharge tube 1 is connected to a vacuum pump 18 by means of a tube 16 led through a Dewar vessel 17 filled with liquid air. One or more devices to be exhausted are connected to the tube 19. After exhaustion has proceeded to a low pressure, a switch 24 is closed and a glow discharge takes place between the anode 7 and the cathodes 2, 3, this discharge being initiated more easily than in the absence of the magnetic field and the discharge current having a higher value. This current is measured by the extent of the glow on the cathode 13 of the discharge tube 14 which is a measure of the gas pressure. The tube 13 may be provided with a scale calibrated in gas pressure. Different measuring ranges may be obtained by altering the'strength of the magnetic field, which for this purpose may be produced by a coil. In order to prevent damage to the insulation by metal particles disintegrating from the cathode plates, the pinches 20 are provided with short glass tubes 21 surrounding the lead-in wires 5, 8, and annular contractions 22 are made in the wall of the vessel. The cathode is preferably of zirconium, thorium, or other metal showing only small disintegration. An earthed metal casing 23 surrounds the apparatus and is provided with a window through which the tube 14 may be viewed. In alternative constructions of the glow discharge tube 1, the angle between the magnetic and electric lines of force is greater than 40, preferably about 90, in at least part of the discharge space, for instance flat parallel electrodes or coaxial cylindrical electrodes may be arranged so that the magnetic lines of force are perpendicular to the shortest connecting lines between the electrodes. The current through the discharge tube 1 may operate a relay controlling the vacuum pump.