GB486767A - Gaseous discharge tube cathode - Google Patents

Abstract

486,767. Thermionic valves. EDWARDS, D. V., and SMITH, E. K. Dec. 4, 1936, No. 33294. [Class 39 (i)] In a discharge device containing gas or vapour at a pressure of from À005 to 1 mm. the cathode is so arranged that the perpendicular distance from the emissive surface, or a sufficient portion thereof to carry the required load, to any other surface is not less than the mean distance traversed bv an electron before undergoing an ionizing impact. For a tube filled with argon at not less than À4 mm. or with xenon at not less than À2 mm. the requisite distance is about 1.2 cms ; with À05 mm. of argon or À025 mm. of xenon a distance of 2À8 cms. is necessary. If the cathode is plane or a simple cylinder the distance is measured from the actual cathode surface, but in other cases an equivalent surface, from which the electrons may be imagined to have been emitted with zero velocity is employed. The Figure shows a valve embodying such a cathode in the form of a cylinder 3, the diameter of which is equal to the distance in question, coated internally with emissive material and heated by an external heater 4. A number of cylindrical insulated screens 5 surround its cathode. The pressure in the device should be above the pressure at which the are drop suddenly decreases with increase of pressure and above that at which the permissible anode voltage increases as.the pressure decreases. A cathode consisting of a number of helical filaments arranged in a circle is also described.