Constantan is an alloy of copper and nickel characterised by an electrical resistance virtually independent of temperature and therefore suitable for building electrical resistors. As copper and nickel are two perfectly isomorphous metals, they can be mixed in the liquid state in any proportions. Depending on the proportions of copper and nickel, the conductivity curve of the alloy has a distinctive shape with a very pronounced minimum around the 50% mark and a very rapid increase as it approaches pure metal. In practice, an alloy of 60% copper and 40% nickel is generally used with a specific resistance of 0.5 Ω/mm²/m.

These two metals can also form an alloy by simple diffusion without passing through the liquid phase. The experiment was conducted by Bruni who heated a copper wire on which a layer of nickel had been deposited. The resistance of the ensemble increased very markedly following the formation of constantan. NB: Constantan is the generic name for alloys of copper and nickel whatever the percentages of the two metals. The Constantan used in type T thermocouples (Copper/Constantan) is not the same as that used in type J thermocouples (Iron/Constantan).