Improvements in or relating to electric heaters

摘要

1,112,274. Resistance heating elements. ARMSTRONG CORK CO. 17 June, 1965, No. 25694/65. Heading H5H. [Also in Division B5] A large area electric heater comprises a semiconductive layer, having a uniform thickness d inches and a resistivity R a in the range 4 x 103 to 4 x 107 ohm-inches, sandwiched between two electrodes at least one of which is a metal foil having a resistivity R s ohm-inches, a thickness t in the range 0À0001 to 0À01 inch, and a length L inches. The heater is constructed so that when a voltage V is impressed across the electrodes, current flows therebetween in a direction perpendicular to the planes of the electrodes, and the power per unit area P = V2/R a d does not vary more than 20% over the area of the heater. This is achieved by choosing t = KR s L2/R a d where K depends on the position and form of the power connections which extend across the width of the electrodes, and is in the range 4 to 2000. The power connections may be at corresponding or opposite ends of the electrodes and may be formed by extensions of the electrodes or by strips applied over the edges of the electrodes. One of the electrodes may be a rigid plate, so that the heater forms a food-warming or cooking plate, or the heater may be a flexible sheet for use in floors, walls, ceilings or roads. The heater may comprise two semi-conducting layers sandwiched between three electrodes, both outer electrodes being earthed. The electrodes may be of aluminium, copper or stainless steel and the semi-conducting adhesive layer of carbon black or graphite mixed with an epoxy resin or thermoplastic material. A suitable epoxy resin is the reaction product of epichlorohydrin and a bis-phenol which is mixed with a hardening agent or catalyst, e.g. amines or amides, and melted or applied as a film to one or both electrodes which are then pressed together and heated to cure the resin. Suitable thermoplastic materials are polyvinyl acetate, vinyl chloride - vinyl acetate copolymers, vinyl acetate-ethyl acrylate copolymers, ethyl acrylate - methyl methacrylate copolymers, and acrylic ester polymers. Conducting ceramic materials may also be used for the semi-conducting layer. Specific examples are given.