The thermocouple is composed of two different wires, welded together to form a knot. At the other end of the signal line, usually as part of an input instrument, is another connection, referred to as the reference join point. The heating induction junction produces a thermoelectric potential (emf) proportional to the temperature difference between the two connections. This millivolt level electromotive force indicates the temperature of the sensing tip when compensating for the known temperature at the reference junction. Published millivoltmeter assumes that the reference is in 0 ° C. Thermocouples are simple and familiar. However, due to the need for special extension and reference joint compensation, it is very complicated to design them into systems.
