Patent GB-727885-A (GB)
Title: Improvements in electric discharge lamps comprising thermionic electrodes
IPC: H01J61/067
Filed: 1950-07-13
Abstract: 727,885. Discharge lamps; electrode materials. GENERAL ELECTRIC CO., Ltd., and FRANCIS, V. J. July 13, 1951 [July 13, 1950], No. 17570/50. Class 39 (I). An electric discharge lamp of the type wherein each electrode which acts as a cathode consists of a metal carrier supporting a store of electronemissive material arranged to be heated at least partly by the discharge, said electron emissive material consisting substantially wholly of one or more metal oxides in admixture or in chemical combination with each other, has the store of electron emissive oxide material for each cathode electrode including both a quantity of relatively soft oxide material which has initially a relatively high activity but a relatively short life, and a quantity of relatively hard oxide material which has initially a relatively low activity but a relatively long life and whose activity increases during the initial period of use of the lamp, the oxide materials occupying spacially distinct positions relative to each other and to the metal carrier part of the electrode and being arranged so that activation of the electrode is initially effected wholly or mainly by the relatively soft oxide material but eventually wholly or mainly by the relatively hard oxide material. The relatively hard oxide material may consist of sintered alkaline earth oxide or thorate and the relatively soft oxide material may consist of non-sintered alkaline earth oxide. The hard oxide material may be in the form of a rod enclosed within a spiral of refractory wire, for example, tungsten, and the soft oxide material provided in the form of a coating either on the rod or on the wire. In another construction the hard oxide material is in the form of a coating on a wire electrode and the soft oxide material is applied as a coating on top of the hard oxide coating. In the construction shown in Fig. 2 the electrode 2 is mounted on support wires 8, 9, each provided with a sleeve of glass 12, 13 in sealing contact with the wires and each having a ceramic bead 14, 15 placed over the joins between the electrode 2 and the wires 8, 9. In the manufacture of the electrode a tungsten wire of 0.05 mm. diameter is wound into a primary coil on a molybdenum mandrel of 0.15 mm. diameter to a pitch of 0.1 mm. and the coil together with the mandrel is then wound on a tungsten mandrel of 0.25 mm. diameter to a pitch of 0.375 mm. to form a coiled coil. The coiled coil, together with the molybdenum and tungsten mandrels, is cut into lengths and each length is again coiled so as to form a helix 16, having a diameter a of 5.5 mm., an overall length b of 7.5 mm. and a pitch c of 3.5 mm. The molybdenum mandrel is subsequently dissolved out but the tungsten mandrel is retained. The electrode is then double coated as referred to above, specific examples being described. Completed electrodes 2, 3 are sealed into the ends 4, 5 of the limbs 6, 7 of a U-shaped envelope 1, Fig. 3, provided with a mixture of 99.5 per cent by volume of neon and 0.5 per cent by volume of argon at a pressure of 9 mm. of mercury, and 1 gr. of sodium. The envelope 1 is provided with a bayonet type cap 181 of porcelain, and the support wires 8, 9 and 10, 11 respectively, are joined together outside the envelope and connected to contact plates mounted on the cap. An insulated metal starting strip 19 is connected to one of the contact plates. The envelope 1 is mounted within a cylindrical glass outer jacket 20 comprising an inner glass envelope 21 and an outer glass envelope 22, both of these envelopes being closed at their ends 23, 24. The other ends of the envelopes are sealed together and the space between the envelopes is evacuated. A porcelain cap 25 is attached to the jacket 20 by adhesive, the cap being provided with an aperture through which the envelope 1 is arranged to pass and a fixing screw 27 being mounted in the cap 25 and bearing on the cap 181. Specifications 408,062 and 487,699 are referred to.