Patent GB-810896-A (GB)
Title: Electric heating units and methods of making same
IPC: H01B7/16
Filed: 1957-02-21
Abstract: 810,896. Sheathed electric heating resistors. GENERAL ELECTRIC CO. Feb. 21, 1957 [Feb. 27, 1956(2)], No. 5888/57. Class 39(3) In making electric heaters having a helical heating wire embedded in a refractory packing in a metal sheath with a member of beryllium, magnesium, aluminimum or titanium metal treated in the sheath to become wholly or partially oxidized to an amorphous metal oxide, the oxidation is carried out in an atmosphere of pure oxygen introduced into the sheath at an elevated temperature below the ignition temperature of the metal oxidized e.g. 1075 ‹F in an autoclave at a pressure of 70 lbs per sq. in. Thus the heater wire or two such wires 112,113 and a wire of magnesium &c. are wound in bifilar relation with a turn of magnesium wire between the turns of the heater wire so that after oxidation, the turns of the heater wire are separated from each other by the amorphous magnesium oxide 115, the space 114 within the heating helix and the space 116 between the helix and the sheath being filled with separately added crystalline-magnesium oxide, Or a heating wire of nickel chromium may bewound on a rod of magnesium and placed in the sheath which is then filled with crystalline magnesium oxide. A porous plug of frangible chalk-like material of porcelain material is placed in non air-tight relation in each end of the sheath, the diameter of which is reduced by several rolling passes which crush the plugs, and on oxidation of the magnesium by oxygen in the autoclave, a. core of amorphous magnesium oxide is formed within the helical heater wire, the conversion of magnesium to oxide being complete or only 10 per cent. After cooling and further rolling the ends of the heater are removed leaving a terminal pin protruding at each end. A glass plug is cast in place at each end to seal the space between the terminal pin and the sheath. Several heater assemblies are treated simultaneously in the autoclave, which is connected to a vacuum pump for removing air from the porous refractory material before oxidation of the magnesium. The heater may then be bent to form a hot-plate and its top surface is flattened. In another modification, a helix of magnesium is fitted loosely against the inner surface of the sheath, which is thus finally lined with amorphous magnesium oxide. In Fig. 6 the nickel-chromium wires 112, 113 are secured together e.g. by welding to a threaded terminal pin 121 at one end and at the other end to separate terminal pins 117, 118 so that the heating wires may be used separately or together. In Fig. 8 two heating wires 142, 143 are shown wound in bifilar relation with interposed turns 153 of magnesium wire. Heating wire 142 is welded at its ends to projections on end terminals 148, 149b, the end turns of the wire being insulated from notches in end terminals 147, 149a to which the wire 143 is similarly connected, so that the wires are in separate circuits. In assembling this heater, the pair of terminals at each end is formed by two metal parts fitted together at their outer ends, and the interfitting parts are cut off when the end parts of the sheath are removed after rolling. The remaining ends of the terminals are then bent outwardly and the glass plugs 150 are then cast in place. The sheath is reduced from a diameter of 0.312 ins. to one of 0.270 ins.