Patent GB-732772-A (GB)
Title: Improvements in and relating to electric discharge lamps
IPC: H01J61/067
Filed: 1951-04-02
Abstract: 732,772. Supply systems for space discharge apparatus. BRITISH THOMSON-HOUSTON CO., Ltd. April 2, 1951 [April 13, 1950], No. 7575/51. Class 38 (4). [Also in Group XL (a)] An electric discharge lamp and circuit arrangement has two electrodes each comprising a main filament surrounding an auxiliary filament and an activated electron emitting material carried by the main filament or the main and auxiliary filaments, both filaments being connected at one end to one terminal and at the other end to another terminal of different potential of a source supplying heating current, one of said ends on each electrode being also connected to a source supplying the discharge current, and the auxiliary filament having considerably less convolutions and less thermal capacity than the main filament. In the construction shown in Fig. 1 the electrode structure at each end of the lamp comprises a coiled main filament 5 surrounding the coiled auxiliary filament 4, the filament 5 comprising a large number of turns of tungsten wire carrying activated alkaline earth oxides or other compounds and the filament 4 comprising a small number of turns of tungsten wire which may be coated with electron-emissive material. In the construction shown in Fig. 4, each electrode comprises a helix 10 formed by an inner conductor 7 of 1.5 mil, tungsten with an overwind of three parallel 1.5 mil. tungsten wires 9 which contact the wire 7 at discrete points only. The helix 10 is formed by winding the three tungsten wires 9 around a composite mandrel formed of the tungsten wire 7 and a 10 mil. molybdenum wire, coiling the structure around a 45 mil. steel mandrel, withdrawing the steel mandrel and dissolving the molybdenum wire in a mixture of nitric and sulphuric acids. The helix 10 is coated with barium and strontium carbonates decomposed to the oxides by placing the electrode in an evacuated space and heating it. The plurality of tungsten wires 9 can be replaced by a single tungsten overwind wire of larger size or by a flat ribbon wire. In the fluorescent lamp and circuit arrangement shown in Fig. 5 the glass envelope 12 contains a rare gas such as argon at a few mms. pressure together with a small quantity of mercury and two electrodes 11, 11<SP>1</SP> constructed as described above. Auxiliary anodes 13, 13<SP>1</SP> may be associated with the electrodes 11, 11<SP>1</SP> to protect them from electron bombardment. The lamp is included in a circuit 14, 14<SP>1</SP> connected across the output terminals of a high leakage reactance transformer 15 having a primary winding 16 with a pair of input terminals 17, 17<SP>1</SP> connected to a source of A.C. The output circuit of the transformer 15 is constituted by one side of primary winding 16 or by one side of a secondary winding 18 which is loosely coupled to the primary winding and connected to the other side thereof. A pair of low voltage secondary windings 19, 19<SP>1</SP> are connected to the electrodes 11, 11<SP>1</SP> for supplying heating current thereto. An auxiliary starting electrode 20 which may be either a thin strip of metal such as aluminium or a conductive coating either painted on the envelope or produced by the application of stannic chloride thereto is located longitudinally along the outer wall of the glass envelope. The starting electrode 20 is connected, in series with a limiting resistance 21, to one side of an auxiliary winding 22 on transformer 15. The other side of auxiliary winding 22 is connected to one side 14<SP>1</SP> of the circuit 14, 141. The transformer 15 is designed so that windings 19, 19<SP>1</SP> provide just sufficient current to electrodes 11, 11<SP>1</SP> to heat the wire 7, Fig. 4, to a temperature in the range from 550‹ to 900‹ C. The heating current is a small fraction of the discharge current, for example less than 1 watt, and can be maintained during the normal running of the lamp. Alternatively the heating current may be reduced in the same proportion as the reduction between the starting voltage applied across the lamp electrodes and the normal operating voltage such as in a circuit where a simple series inductance is used in the discharge circuit as a ballasting device and the transformer providing heating current to the lamp electrodes is connected on the lamp side of the series inductance rather than on the line supply side. By such an arrangement it is possible to obtain a reduction in. the heating wattage in the ratio of approximately 2 to 1.