Gaseous Tubes for Lighting

Abstract

THE study of the discharge of electricity through gases has led to many important practical developments, the latest being the invention of vacuum tubes for lighting. If discharge tubes are filled to the same gaseous pressure with different gases such as helium, neon, argon, hydrogen and nitrogen, it is found that a certain definite voltage must be applied to the terminals of the tube before luminous effects are produced. This voltage depends on the nature of the gas and the pressure in the tube. In general, neon produces a luminous effect at lower voltages than the other gases and then comes helium. In some cases a mixture of gases becomes luminous at a lower voltage than any of the pure gases forming it. In a lecture to the Preston Scientific Society given on December 2, 1932, J. N. Addington gave an interesting account of the progress made in developing gaseous tubes for lighting and signalling purposes. In an Osglim lamp, which takes only a small fraction of the power taken by a filament lamp, the electrodes are of nickel or iron placed about 2 mm. apart and the neon gas filling has a definite low pressure. At 160 volts the lamp becomes luminous. These lamps operate from alternating current mains. Neon valves which operate traffic signal devices have now been developed atthe Prestonlampworks of Messrs. Siemens Brothers and Co. The red neon signs which are now familiar to all dwellers in cities have iron or nickel electrodes and the illumination is produced from the positive column. The intensity of illumination per unit area is relatively low in these lamps, which are called cold cathode tubes. In ‘hot cathode tubes’ currents nearly a hundred times as great can be obtained. The electrodes are coated with mixtures of alkaline earths and have a very high electronic emission when heated to 1,200° C. The use of metallic vapours like sodium, cadmium and magnesium has still further increased the efficiency of these tubes, but the colours at present obtained leave much to be desired when compared with daylight.