Patent GB-520183-A (GB)
Title: Improvements in and relating to metal vapour electric discharge devices
IPC: H01J13/12
Filed: 1938-10-10
Abstract: 520,183. Discharge apparatus. BRITISH THOMSON-HOUSTON CO., Ltd. Oct. 10, 1938, No. 29337. Convention date, Oct. 9, 1937. [Class 39 (i)] A "pool" type discharge device having a maintaining electrode has a hollow shielding member extending from the cathode surface to and around the maintaining electrode and separating the maintaining arc from the main discharge space, excepting through one 'or more restricted openings at the cathode surface, and means for anchoring the maintaining arc spot in the vicinity of the openings during the use of the device, which means are in contact with the cathode surface. In the form shown in Figs. 1 and 2, a metal envelope 10 encloses an anode 12, which may form part of the envelope, a cathode 11 of mercury, cadmium or bismuth, a high-resistance starting electrode 33 and maintaining,anodes 17, 18. Voltages are supplied through a transformer 35 to the anodes 17, 18 and to the starting electrode 33 through a rectifier 37, resistance 36<SP>1</SP> and relay 36 which is opened by the current to the maintaining anodes. The maintaining arc discharge is shielded from the main discharge by a refractory member 21, Fig. 2, of metal, quartz or alumina and a member 40 of molybdenum or tungsten. Passages 41 in the member 40 allow the main arc to be struck but are sufficiently small to prevent the passage of the cathode spot of the maintaining arc, the 'cathode spot of the main arc being found externally of the shields 21, 40. The semi-cylindrical electrodes 17, 18 and the electrode 33 are supported by a header 20 which is secured within the cylinder 32 by a glass or metal fillet 23, and the leading-in wires 25 are sealed to tubes 28 of an alloy of iron, nickel and cobalt by glass 27; extensions of the glass seals insulate the wires from the cathode and are screened by tubes 31 of metal, glass or quartz. Rods 38 support the screen 40, 21. In another form, the tungsten lead 53, Fig. 6, of the high-resistance starting electrode 54 forms the maintaining anode and the alumina or glass screen 50 has a metal extension 51 of reduced segmental cross-section which has narrow apertures 52. In a further form, the insulating screen 60, Fig. 8, is supported by brackets 71 in such manner that it is spaced at 63 from an annular insert of sintered carburized molybdenum 68 which projects from, and is wetted by, the mercury. A baffle 70 is provided in the mercury. In the form shown in Fig. 9, the holding arc is struck by a sparking electrode 75 energized through a transformer 77, the maintaining anodes 78, 79 being energized from a transformer 80 which is centretapped to,the cathode 76 through a relay 82 so set as to cut out the sparking electrode while the maintaining arc is running. A baffle 84 separates the electrodes 78, 79. To control the point of ignition of the main arc in each cycle, an electrostatic control electrode may be used in the form of a metal band mounted on the insulating shield adjacent the discharge openings therein. A negative potential is supplied to the band to prevent ignition and positive potentials are supplied thereto through a transformer and a phase shifter to start the main arc. The passages through the shield may be ' closed by a pair of external cylindrical grids which dip into the mercury so that the magnitude of the control potentials applied to the band may. be reduced. Fig. 14 shows a form in which the insulating enclosure 134 is secured to a graphite or alumina screen 136 a channel in which is closed by molybdenum barriers 140, 141 provided with slots which are offset with respect to each other. The cathode spot may be kept near the barriers by a magnetic field. A control electrode 146 is shown on the screening member 134 but it may be placed on or within the shield 134 or between the barriers. Specification 494,404 is referred to. Reference has been directed by the Comptroller to Specification 456,243.