Constantan

Constantan is a copper-nickel alloy whose electrical resistance hardly changes with temperature. But that also makes the material tricky to weld: because copper and nickel react differently to heat, the weld seam tends to crack and distort if you don't work cleanly. Best to use a filler material that matches the composition, and weld with calm, even heat input. If you make the seam too hot or cool it too quickly, you'll degrade the alloy's electrical properties – so better to work slowly and check the part after welding.