Seek of Berlin noticed in 1824 that an electric current flows in a circuit consisting of two dissimilar metals if one junction, say A, is kept cold and the other, say B, is heated as shown in. This experiment I remarkable because no cell is required for sending current through the circuit.

At the hot junction B, the current flows from copper to iron while at cold junction A the current flows from iron to copper. The current is called thermo-electric current and the combination of two dissimilar metals is known as thermo-couple. The phenomenon of production of e.m.f. by thermo-couple when the two junctions are kept at different temperatures is known as Seeback effect, E.M.F., so produced is known as thermo electric e.m.f.

E.M.F. of thermo-couple depends upon

(i)    the nature of material of metals and
(ii)    the difference of temperatures of two junctions.

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