Metal oversheathed electrical resistance heater

A well heater comprising:

• a tubular metal oversheath encasing both a plurality of metal surfaced heating strands and at least one other metal surfaced strand;
• said metal surfaced strands being at least substantially parallel to each other and the oversheath;
• said oversheath being compressed radially inward to establish metal-to-metal contact between it and the strands within an assembly which is spoolable and is substantally impermeable throughout its length;
• said heating strands including a plurality of electrically conductive cores which cores are capable of resistive heating, are surrounded by solid insulation contained within a metal sheath and are positioned and interconnected in an arrangement providing a combination of electrical resistance per unit length of the well heater such that the heater is capable of generating heat at different rates along different portions of its length;
• and
• means for supplying electric current to said heating strands.