A small amount of electricity can be generated from heat by connecting two dissimilar metals and heating the spot where they are joined. Metals such as copper and constantan, a copper/nickel alloy, or iron and nickel are typical pairs.
Each metal reacts to the heat differently, causing a different movement of electrons between the two. This device is called a thermocouple and the spot where the two metals are connected is called the junction.
In an iron-nickel thermocouple, applying heat to the junction force the electrons to move from the iron to the nickel, resulting in a small but measurable voltage. These voltages are typically in the thousandths of a volt, or millivolts.
This thermoelectric process is frequently used in furnaces to sense the presence of heat and to hold the gas valve open as long as the heat is present, and to allow it to close if the flame goes out. It is a simple way to measure temperature but not a very efficient way to generate any significant quantities of electricity.