GB591738A - A tool for localised electric high frequency heating of insulating materials - Google Patents

Abstract

591,738. Dielectric heating. BRITISH INSULATED CABLES, Ltd., QUAYLE, J. C., and JONES, P. May 12, 1945, No. 11969. [Class 39 (iii)] In a tool for spot heating insulating materials by dielectric losses, a central conductor 8, 9 and a barrel 1, 3 carried by a handle or other support 2, constitute two telescopic, co-axial conductors which are connected to a high-frequency electric supply, e.g., 1,000-10,000 megacycles per second, and form at one end two concentric electrodes 23, 38 to be pressed on the work, whereupon the tool telescopes against the action of a controlling resistance such as a dashpot 33, so that the effective electrical length of the barrel reduces and passes through one or an odd multiple of quarter of the wavelength of the electric supply. At this value the electric stress in the work is a maximum. Conveniently, the barrel has an initial length greater than one or an odd multiple of quarter of the wavelength by less than quarter of the wavelength, and.telescopes through approximately quarter of the wavelength. In the example shown, the barrel comprises telescopic elements 1, 3 guided by metal collars 4, 6 and by a piston 5 carried by the element 3. The ends of a metallic bellows 31 containing oil are closed by the piston 5 and the wall 32 of an air chamber 30 into which the oil can flow through an annular plug 35. Thus different plugs can be used to regulate the rate of telescoping of the tool. The oil can return through a discharge valve 37 as the tool returns to its normal length. The central conductor comprises an element 9 slidably received in an element 8 which is fixed to the element 1 by an insulating spider 12 having arms extending through slots 14 in the element 3. Screens 41 for the slots 14 are carried by the element 1 in electrical contact with the element 3. The element 9 includes the electrode 23 which projects beyond the electrode 38 and retracts, on engaging the work, against the action of a spring 21. A spring 18 between insulating annuli 11, 19 acts to return the tool to its normal length. The high-frequency supply is connected to co-axial conductors 24 and 29 attached respectively to the elements 8 and 1. The conductor 24 extends through apertures in the elements 1, 3. The mean diameter of the electrode 38 is preferably one quarter of the wavelength, e.g. 2 inches..The ratio of the diameters of the adjacent electrode surfaces may be 2 : 1.