Thermocouples:

A thermocouple is a device used to convert heat energy into a voltage output.  An thermocouple consists of two different category of metal joined at a junction in the Figure.

Figure:  Production of a DC Voltage Using a Thermocouple

As the junction is heated, the electrons in one of the metals gain sufficient energy to become free electrons.  A free electron will then migrate across the junction and into the other metal.  This displacement of electrons generates a voltage across the terminals of the thermocouple. A combinations used in the makeup of a thermocouple involve:  iron and constantan; copper and constantan; antimony and bismuth; and chromel and alumel.

Thermocouples are commonly used to measure temperature. The voltage generate causes a current to flow by a meter, that is calibrated to denotes temperature.