high temperature coefficient of resistance an alloy of about 6 percent by weight nickel and 94 percent by weight copper known as lohm has both a relatively low resistance and a low temperature coefficient of resistance this results in a more simple power supply circuitry and less of a tendency to form hot spots the long element heaters of this invention can be utilized in subterranean oil recovery or coal shale hydrocarbon recovery these types of heaters are often referred to as well heaters a preferred basic heater design for the practice of this invention is described in u s pat no 4 570 715 incorporated herein by reference the well heaters may be of other designs because the present invention is broadly an improved heater core metallurgy which can be utilized in numerous long heater designs the reason for the decreased tendency to form 34 hot spots 34 which result in premature heater core failures can be seen from comparing the 34 normalized resistance 34 of different potential heater core materials the normalized resistance is the resistance of a metal at a temperature divided by the resistance of that metal at room temperature because resistances of metal change almost linearly with temperature a metal with a lower normalized resistance at an elevated temperature will have a much lower relative change in heat output if the temperature of the core increases normalized resistance of nickel and copper at 800 u00b0 c are about 5 8 and about 4 8 respectively the normalized resistance of 34 30 alloy 34 at 800 u00b0 c is about 2 2 the normalized resistance at 800 u00b0 c of an alloy of 6 nickel and the.