Fe-CuNi thermocouple (type J) is also named Fe- Constantan thermocouple. The positive leg Fe(JP) is made by pure Iron, and the main chemical composition of negative leg Constantan (JN) is Cu: Ni≈ 55%: 45%. Although they are all called Constantan, JN does not give the same EMF output curve as EN and TN, it cannot be instead by other Constantan.