Patent GB-669036-A (GB)
Title: Improvements in and relating to electric discharge lamps
IPC: H01J61/72
Filed: 1947-12-18
Abstract: 669,036. Discharge lamps. BRITISH THOMSON-HOUSTON CO., Ltd. Dec. 18, 1947 [Dec. 21, 1946], No. 33458/47. Class 39 (i). A positive column gaseous electric discharge lamp comprises a sealed tubular envelope 1 containing an ionizable atmosphere at a pressure of a few mm. of mercury, an anode 6 and an activated thermionic cathode 7 mounted at opposite ends of the envelope, and leadin conductors 3, 4 and 5 to the anode and cathode sealed through the end of the envelope which is nearer the anode, wherein the anode surface is of non-electron emissive material, particularly a non-metallic material, to pass substantially no current when connected as a cathode, the construciion being such that the operating arc voltage fs below the voltage which would be required tor a discharge between the anode 6 and the cathode conductors 4, 5 which are bare and pass near the anode, so that arcing between the anode and base conductors is prevented, and the starting and operating voltage gradient being such as to require the anode to conduct an appreciable amount of current if it were to operate as cathode. The anode may comprise a disc or block of graphite or carbon or a metal body coated with a carbonaceous material, e.g. soot or carbon. The lead-in conductors 3, 4, 5 may be made of the alloy of iron, nickel and cobalt known as Fernico and may be coated with a carbonaceous material, e.g. soot or carbon. The lamp can be made a full-wave rectifying type by employing two anodes arranged side-by-side and connected for operation during alternate half-cycles. The cathode 7 may be made of a coiled or coiled-coil structure of tungsten wire overwound with finer tungsten wire and impregnated with alkaline earth oxides as disclosed in Specification 569,685, [Group XI]. The leads 4, 5 may be twisted together or otherwise connected outside the envelope if it is desired to short-circuit the cathode 7. One of the leads can be omitted and the cathode heated for activation by a known high-frequency coil. The envelope can be made of glass marketed as 9741 by the Corning Glass Works when the lamp is to be used as a source of germicidal radiations of 2537 A or the inner surface of the envelope may be coated with calcium tungstate or cerium-activated calcium phosphate which convert the radiations of 2537Š generated by a mercury vapour discharge to ultra-violet radiations of longer wave length. The envelope contains argon at a pressure of about 1-10 mm., preferably 6 mm., together with a small quantity of mercury which during operation of the lamp attains a pressure of about 10 microns. The lamp can be operated from a leakage reactance auto-transformer 8 connected to the terminals 9, 10 of an A.C. source, the transformer secondary being connected to the cathode lead 4 and anode -lead 3. The cathode may be separately heated by connecting its leads 4, 5 across a separate source of potential (not shown), as by tapping into the transformer 8, in which case a switch may be provided in the circuit to lead 5 whereby to preheat the cathode only during starting the lamp.