GB184446A - Improvements in and relating to electron discharge devices and electron emitting cathodes therefor - Google Patents

Abstract

184,446. British Thomson - Houston Co., Ltd., (Assignees of Langmuir, I.). Aug. 11, 1921, [Convention date]. Vacuum tubes; low-pressure metallic-vapour apparatus.-The cathode comprises tungsten or other refractory metal, thorium, or a compound such as thoria and carbon. The introduction of carbon, preferably in the proportion of twothirds per cent, and, in any event, below 3 per cent, prevents reoxidation of the active thorium, and reduces thoria to the active state. According to one method, the filament is prepared by squirting a mixture of tungsten, thoria, and a carbonaceous binder. According to another method, a tungsten filament 2, containing three-quarters per cent of thoria, is mounted in a bulb 1 with an anode 3 of tungsten or molybdenum. The bulb is connected with a pump through a tube 6, and with a bulb 7 containing a substance giving a low pressure of carbonaceous vapour, such as anthracene or naphthalene. The bulb 7 may be surrounded by a receptacle 8 containing a freezing-agent to reduce the vapour pressure; in the case of naphthalene, the temperature of the bulb 7 is reduced to -20‹ to -30‹C., the vapour pressure being 0.1 to 0.01 micron. The filament is heated at about 2300‹ absolute for a few minutes in contact with the hydrocarbon vapour, tungsten carbide being formed. The heating is continued until the resistance of the filament shows an increase of 15 to 25 per cent above that of the filament when cold; this condition corresponds with the presence in the filament of about two-thirds per cent of carbon. The vapour is then removed from contact with the cathode by sealing a contraction 9, or by cooling the bulb 7 by liquid air. Gas is removed from the anode by electron bombardment from the cathode, and withdrawn by the pump. A non-carbonaceous reducing material, such as sodium or potassium, may be introduced into the bulb 1, or, when some positive ionization is desired, caesium may be provided by adding a mixture of caesium halide and metallic calcium as a paste to some part of the bulb. When ionization of residual gases or vapours is not desired, a reagent having a lower vapour pressure than the alkali metals may be used, for example, magnesium, calcium, or strontium. For this purpose, a tungsten filament 11, containing magnesium, may be mounted in the bulb 1, the magnesium being volatilized and deposited on the walls of the bulb when the filament is heated. After sealing off the bulb at a contraction 10, the filament may be heated again to absorb any oxygen or water vapour driven off from the glass. The cathode is then heated for a short time to about 2700‹ absolute to volatilize any thorium oxide on its surface. Further heating for a few minutes at 2200‹ absolute causes reduction of thoria by carbon, the active thorium being brought to the surface. In use the cathode is heated to a temperature below 2200‹ absolute, for instance 2100‹. Specifications 7655/06 and 148,132 are referred to.