Thermoelectricity:

A few materials readily provide up their electrons and others readily accept electrons.  For instance, when two unlike metals such as zinc and copper are joined together and a transfer of electrons could take place. An Electron will leave the copper atoms and enter the zinc atoms. Zinc gets a surplus of electrons and becomes negatively charged.  The copper loses electrons and takes on a positive charge. This makes a voltage potential across the junction of the two metals.  To make them release and gain electrons the heat energy of normal room temperature is sufficient, causing a measurable voltage potential.  More electrons are released as more heat energy is applied to the junction, and the voltage potential becomes greater, as display in Figure.  The charges will dissipate and the voltage potential will decrease when heat is removed and the junction cools. This procedure is known as thermoelectricity. A device such as this is commonly referred to as a "thermocouple."

The thermoelectric voltage in a thermocouple is dependent upon the heat energy applied to the junction of the two unlike metals.  Thermocouples are hugely used to measure temperature and as heat-sensing devices in automatic temperature controlled equipment.

Figure: Heat Energy Causes Copper to Give up