A closed circuit, which is made of two different metals, is called a thermocouple.
Due to the different metal free electron gas density is not the same, in the welding of two kinds of metal free electron interdiffusion between, resulting in two metallic contact occurs at a potential difference, this is contact electromotive force.
Contact electromotive force in addition to the two kinds of metal properties related to the temperature, in the same case, the two joints at the same electric potential value, the direction of the opposite, the total electric potential is zero. If the two junction temperature is different, the two electric potential value is different, the total electromotive force is not zero, the closed circuit will appear current, which is caused by the temperature difference is called the current.
Thermocouple temperature measurement method is: make a joint of known temperature, another joint into the object whose temperature is to be measured, measured by thermocouple thermoelectric current, inferred from the measured temperature.