A thermocouple is a device that is used for measuring temperature. It basically consists of two dissimilar metals , that are joined to form a junction, which when heated ,produce a thermoelectric voltage.
This voltage changes as the temperature increases or decreases. Thermocouples are self powered and require no external form of excitation.
Thermocouples are widely used in science and industry applications include, temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes