Seebeck Effect

If two different (dissimilar) metals are joined at two points to form a single closed loop, and these two junctions are then held at DIFFERENT temperatures, a small emf is produced within the loop and a current flows through the metals purely because of this temperature difference. This emf is called the thermo-emf, the phenomenon itself is the Seebeck effect, and the particular pair of dissimilar metals forming the two junctions is called a thermocouple.

The DIRECTION in which the current actually flows depends on which two metals are paired. For an antimony-bismuth thermocouple, the current flows from antimony to bismuth AT THE COLD junction (remembered as the 'ABC rule': Antimony to Bismuth at the Cold junction); for a copper-iron couple, by contrast, the current flows from copper to iron at the HOT junction. In both cases the effect is reversible -- if the hot and cold junctions are physically interchanged (i.e. which junction is heated and which is cooled is swapped), the direction of the current reverses too.

If the cold junction of a thermocouple is held at $0^\circ$C and the hot junction at $T^\circ$C, the thermo-emf generated is given, to a good approximation, by

$\varepsilon = \alpha T + \frac{1}{2}\beta T^2$ …