271. Seebeck discovered in 1821 that if in a closed circuit of two metals the two junctions of the metals are at different temperatures, an electric current will flow round the circuit. If, for example, the ends of an iron and of a copper wire are soldered together and one of the junctions is heated, a current of electricity will flow round the circuit; the direction of the current is such that the current flows from the copper to the iron across the hot junction, provided the mean temperature of the junctions is not greater than about 600° Centigrade.

The current flowing through the thermoelectric circuit represents a certain amount of energy, it heats the circuit and may be made to do mechanical work. The question at once arises, what is the source of this energy? A discovery made by Peltier in 1834 gives a clue to the answer to this question. Peltier found that when a current flows across the junction of two metals it gives rise to an absorption or liberation of heat. If it flows across the junction in one direction heat is absorbed, while if it flows in the opposite direction heat is liberated. If the current flows in the same direction as the current at the hot junction in a thermoelectric circuit of the two metals heat is absorbed; if it flows in the same direction as the current at the cold junction of the circuit heat is liberated.