Sometimes resistance wire is insulated by ceramic powder and sheathed in a tube of another alloy.
Such heating elements are used in electric ovens and water heaters, and in specialized forms for cooktops.
Constantan (Cu55Ni45) has a low temperature coefficient of resistivity; and as a copper alloy, it is easily soldered.
Balco (Ni70Fe30) and similar alloys have very high, but more linear, temperature coefficients of resistivity, making them suitable for sensing elements.
The resistivity of amorphous carbon actually has a range of 3.8 – 4.1 × 10−6 Ω m. Trade names include:[3]