Thermocouple
Electrons, when heated, will begin to cross the junction on their own. Because of the different properties of the different metals, the electrons will lose potential energy and gain kinetic energy, just like a ball rolling over a hill to a lower area. Although the voltage created by a thermocouple is very small (in the millivolt range), many thermocouples can be hooked together to make a larger voltage. This is called a thermopile.