Patent GB-716621-A (GB)
Title: Improvements in or relating to the manufacture of cold-cathode gas filled electric discharge devices
IPC: H01J17/24
Filed: 1951-09-11
Abstract: 716,621. Cathode materials and processing. GENERAL ELECTRIC CO., Ltd., and NEILL, T. R. Sept. 11, 1952 [Sept. 11, 1951], No. 21420/51. Class 39(1) In the manufacture of a cold-cathode gas-filled electric discharge device having an activated cathode, for activating the cathode a readily volatilizible activating material which is also capable of absorbing gaseous impurities is contained within a hollow tube the wall of which is provided with a longitudinal slit through which the vapour of said material can escape, and the tube is included as part of a conducting loop which either constitutes the cathode or is located adjacent to the cathode, and is so arranged that the slit in the tube faces directly towards a part of the envelope wall, and the activating material is volatilized by passing an electric current through said loop so as to heat the hollow tube, said volatilization being carried out in two steps, namely a first step whilst the envelope of the device is highly evacuated, in which step the activating material is deposited mainly on the wall of the envelope so as to act as a getter and very little is deposited on the cathode, and a second step whilst the envelope is filled with gas, in which second step the turbulence induced in the vapour jets, and the rapid cooling of the cathode due to the presence of the gas filling, cause a substantially uniform activating coating to be deposited on the cathode. The tube shown comprises a cathode 5 formed from a length of nickel wire bent to form three sides of a rectangle and welded to a length of getter wire 6 having a barium core 7 enclosed within a tube 12 of aluminium in turn enclosed within an iron tube 13 having a narrow slit to expose the aluminium tube. The cathode wire 5 is also welded to a mounting wire 8 sealed into a glass disc 1 and attached to a terminal pin 2. A nickel wire anode 9 and trigger electrode 10, cranked at the base to obtain correct electrode spacing, are sealed into the disc 1 and attached to terminal pins 3 and 4 respectively. The terminal pins 2, 3 and 4 lie on the circumference of a circle with the cathode and anode pins 2 and 3 situated one at each end of a diameter of the circle and the trigger pin 4 arranged nearer to the cathode pin 2 than to the anode pin 3. After the positioning of the electrodes the glass disc 1 is hermetically sealed into one end of a glass tube 11, the other end of which is closed apart from a narrow exhaust tube (not shown). The envelope is then exhausted and the getter wire heated by high-frequency induced currents so that the aluminium tube splits under the pressure of the vaporized barium which condenses on the wall of the envelope adjacent to the cathode and absorbs any gases which are evolved on heating and are not entirely pumped away. The envelope is then filled with neon at a pressure of 86 mm. of mercury with the addition of 2 per cent of argon, and the exhaust tube sealed off. The getter wire is heated again and a substantially uniform coating of barium is deposited on the cathode surface. The gas filling present in the second heating stage is not necessarily the final gas filling for the tube, and the conducting loop in which the getter wire is included may be closed outside the tube envelope. The invention is also applicable to tubes having either more or less than three electrodes.