Patent GB-395851-A (GB)
Title: Improvements in and relating to electric discharge devices
IPC: H05B41/06
Filed: 1932-05-30
Abstract: 395,851. Discharge lamps. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.- (Assignees of Foulke, T. E. ; 30, Evelyn Place, Nutley, New Jersey, U.S.A.) May 30, 1932, No. 15319. Convention date, May 29, 1931. [Class 39 (i).] In a method of starting and operating an electric gaseous discharge device having a gaseous atmosphere comprising a major component intermixed with a trace of another gas whose ionizing potential is lower than the metastable potential of the major component of the gaseous atmosphere a localized discharge is created by applying the normal operating potential between a pair of adjacent electrodes within the device, so that the radiations from this discharge excite the atoms of the major component to a metastable condition throughout the space between a pair of more widely separated electrodes, these metastable atoms producing an ionization of the atoms of the minor component whereupon the main discharge is initiated between the latter pair of electrodes. Preferably the metastable potential of the .major component exceeds the ionizing potential .of the minor component by less than five volts, and in an example the gaseous atmosphere .comprises neon containing À01-1À0 per cent of argon' at a pressure of less than 10 mm. of mercury. The tube may be operated with either hot or cold cathodes, and in the latter instance a coating of caesium, preferably intermixed with the oxide thereof, produces a discharge in neon at 100 volts direct current with a gas pressure of 3 mm. of mercury. In one form oi the device the tubular envelope 1, Fig. 1, is provided with a cylindrical cathode 2, preferably of nickel, and an anode 3 which may be made of carbon. The auxiliary anode 5, of nickel, tungsten or other material, extends within the cathode and preferably terminates close to the wall thereof. A screw-threaded cap 7 has its centre contact connected through a 500-1500 ohm ballast resistance 9 to the leading-in wire 4 of the cathode, while the inlead of the auxiliary anode 5 is connected to the shell of the cap 7 through a relatively high resistance 10 of 25,000-100,000 ohms. The leading-in wire 6 of the anode 3 is connected to a metal cap 8 which is connected to the shell of cap 7 by a conductor 11 of aluminium foil or the like extending along the surface of the tube 1. The cathode may be of a substance having a low work function, such as caesium or rubidium. In the example a clean nickel cathode 2 is coated with a compound such as caesium carbonate which is reduced to the oxide by heating by high frequency induction during the evacuation of the tube, the evolved gas being drawn off. Pure metallic caesium is then produced by heating.a pellet 12 of a decomposable caesium or other alkaline compound to which a reducing agent may be added, for example a mixture of caesium dichromate and silicon may be used. In a modification the anode may be composed of a sintered mixture of tungsten and barium oxide or the like, and the metal cap at the anode end of the tube is made large enough to accommodate a ballast resistance. In a further modification adapted for alternating current use a cathode similar to the cathode 2 is mounted at each end of the tube, one inlead being connected to one auxiliary anode and, through a ballast resistance, to the cathode at the opposite end of the tube, the other inlead being similarly connected to the other auxiliary anode and cathode. In another modification the tube 20, Fig. 4, contains at one end an anode 21 of graphite, iron, or other material, and at the other end a cathode 23, Figs. 5 and 6, of nickel or like material surrounded by heat shields 24, 25 which are connected thereto and may also be made of nickel. Preferably the outer shield 25 extends beyond the end of cathode 23. The,cathode is divided into compartments by vanes 26 which co-operate to form a central tube enclosing a heater 27 with a heating coil embedded therein in refractory insulating material such as alundum. The inner surface of the cathode 23 and the exposed surfaces of the vanes 26 are preferably coated with alkaline or alkalineearth metals or compounds thereof, for example a mixture of barium and strontium oxides. One terminal of the heating-coil is connected to the leading-in wire 28 and the other terminal to the heat shield 25. The auxiliary anode is in the form of a ring 33 concentric with the heat shield 25, the anode leading in wire 32 serving also as a supporting-wire a part of which is surrounded by a vitreous sleeve 34. The device is adapted for alternating current use by substituting a duplicate cathode for the anode 21. This latter arrangement may be modified by dispensing with one auxiliary anode, the heating coil then carrying the main discharge current. Fig. 9 shows a modification for full wave operation on alternating current, with an anode 43 consisting of a sintered mixture of tungsten and barium oxide. The device starts with a half-wave discharge which changes to full-wave when the anode reaches the electron emitting condition as a result of anodic heating. To ensure that a discharge is not initiated until the cathode emission is sufficient to support the normal positive column discharge a potentiometer 44 is connected between the auxiliary anode 33 and the cathode 23 to limit the potential between them to 30-40 volts. This arrangement may be modified by substituting for the potentiometer a discharge tube in series with the auxiliary anode, so that no current flows through the auxiliary anode circuit until the electron emission from the cathode is sufficient to reduce the voltage drop between the cathode and auxiliary electrode, thereby initiating the auxiliary discharge with consequential initiation of the positive column discharge. Specifications 363,343 and 366,861 are referred to.