If an electrical circuit consists of entirely metallic conductors and all parts of the circuit are at the same temperature, there will be no electromotive force in the circuit and therefore no current flows. However, if the circuit consists of more than one metal and if junctions between two metals are at different temperatures, there will be an EMF in the circuit and a current will flow.
illustrates this effect. The EMF that is generated is called a
and the heated junction is a