620,196. Dielectric heating. AUTIE, E. G., JOHNES, F. W., and APPLIED HIGH FREQUENCY, Ltd., (formerly DIAPLASTICS, Ltd.). Jan. 7, 1947, No. 583. [Class 39(iii)] During spot welding of plastic material by high frequency dielectric heating, the extrusion of plastic radially or laterally outwards from the spot electrode is prevented, the displaced material being retained under the contact face of that electrode. In one embodiment the spot electrode 12, of copper, is surrounded by a pressure member 15 of refractory insulating material which forces the displaced material, during the pressure and welding process, into the recess 14 in the contact face 13. In a further embodiment the pressure member 15 is enclosed in an insulating sleeve 16, and is movable under the control of a spring 17. The spot electrode comprises a fixed copper sleeve 19 containing an inner yielding portion 25 which, during operation, moves against a spring, not shown, to contain the displaced plastic 26. Specification 555,054 is referred to.