Patent GB-488886-A (GB)
Title: Improved device for rectifying alternating current comprising an ionic discharge tube
IPC: H01J17/14
Filed: 1937-05-06
Abstract: 488,886. Discharge apparatus. NAAMLOOZE VENNOOTSCHAP PHILIPS' GLOEILAMPENFABRIEKEN. May 6, 1937, No. 12982. Convention date, May 7, 1936. [Class 39 (i)] In a rectifying device comprising an ionic discharge tube to which alternating current is supplied, the tube has a cold cathode and a low gas- or vapourpressure and the discharge path is traversed by a magnetic field, the electrodes and the field being so arranged that the greater part of the magnetic lines of force traversing the cathode traverses part of its surface both on entering the discharge path and upon emerging therefrom, whereas only a small part of the lines of force so traversing the cathode surface traverses the anode. The tube may consist of two plate-shaped cathodes 3, 4, Fig. 2, having a wire or rod anode 5 arranged normal thereto and a magnetic field due to the magnet 1 is directed along the axis of the anode. Alternatively, the anode may be in the form of a hollow cylindrical member which surrounds the discharge space and may extend substantially the distance between the cathode plates or may be in the form of a narrow ring, and the magnetic field may be produced by a long coil surrounding the tube. In a modification, a wire anode is arranged along the axis of a hollow cylinder forming the cathode and a rotationally symmetrical inhomogeneous magnetic field is produced by a coil which is arranged round the middle of the cathode and of which the length does not exceed onehalf and is preferably only one-fifth of the length of the cathode. Instead of a cathode cylinder, a number of plates surrounding a substantially cylindrical space may be used and the tube axis and the axis of the field may be inclined at a small angle. An alternating magnetic field may be used, which synchronizes with the alternating current supplied to the tube and may be obtained from the same supply. By means of the arrangements disclosed, the electrons in the tube are caused to travel repeatedly between the different parts of the cathode surface, thus producing an increase in the ionization produced, the effect being most marked when the pressure is chosen so that the free path of the electrons exceeds one-tenth of the part of the discharge path extending between the points where the magnetic lines of force enter and emerge. A pressure of 10<-4> to 10<-2> mm. of mercury is suitable and curves accompany the Specification which illustrate the changes in ignition voltage with pressure, both with and without the magnetic field provided according to the invention. Dimensions and operating conditions for a particular tube are given. The tube filling may be neon or mercury vapour obtained from a liquid cathode or the cathode may consist of a metal such as caesium having a low melting-point, preferably less than 30 C. The arrangement may function as an ignition device in a mercury cathode rectifier, as shown in Fig. 6, the mercury cathode 28 and anode 29 forming the two parts of a cathode similar to the parts 3, 4 of Fig. 2, and the ring member 31 forming the anode. A comparatively high voltage 32 which is synchronous with the lower voltage 33 to be rectified is applied between the ring 31 and the mercury cathode 2S and anode 29, and the discharge space is traversed by an axial field produced by the coil 30. Ignition is brought about by a discharge from ring 31 to electrodes 28, 29 and cause sufficient ionization for the starting of a discharge between the cathode 28 and anode 29.