Electric heater construction

Claim 1

In an electric heater, an elongated hollow metal sheath, a plurality of wires extending longitudinally within said sheath including at least one heater wire of a relatively high resistance material and a pair of thermocouple wires of dissimilar metals, insulating material within said sheath for insulating said wires from said casing and from each other, connection means at one end of said sheath for electrical connections between said heater and thermocouple wires and current supply and instrumentation circuits, and junction means at the opposite end of said sheath for interconnecting said pair of thermocouple wires to form a junction for measurement of the temperature thereat, said insulating material being a compacted mineral material, the degree of compaction of said mineral material and the physical properties of said sheath and said wires being such as obtained by assembling wires and mineral material in a sheath and then repeatedly performing drawing and annealing operations to reduce the diameters of the sheath and wires assembled therein to a small fraction of the initial diameter while compacting the mineral material, said elongated sheath being in the form of a helical coil structure with said connection means and said junction means being at opposite axial ends of said helical coil structure, said helical coil structure being directly mounted on an external cylindrical surface of a tubular metal part for direct transmission of heat to the part, an outer tubular member non-directly enclosing said helical coil structure, and said helical coil structure being tightly wound with adjacent convolutions being in direct engagement with each other for uniform transmission of heat to the metal part along the length thereof and to minimize outward transmission of heat from the metal part and from the helical coil structure by convection.