A phenomenon of thermoelectricity.
Electrical voltage is generated at the contacts of two homogenous metals (or semiconductors) if the contacts have different temperatures. This voltage is also referred to as thermal voltage. Due to the temperature difference, electric current flows if the electric circuit is closed.
Thermal voltage is counted positively if current flows from the cold to the hot contact. The basis of the Seebeck effect is that the charge carriers form new groups under the influence of the temperature difference. Thomas Johann Seebeck discovered this effect in 1821 subsequently designing the first thermal element.