Hello

The magnitude and direction of thermo EMF in a thermocouple depends not only on the temperature difference between the hot and the cold junctions but also on the nature of the metals constituting the thermocouple. Few metals forming the series are as below:
Sb, Fe, Cd, Zn, Ag, Au, Sn, Pb, Hg, Pt, Cu, Co, Ni, Bi, Mn
The thermo electric EMF is directly proportional to the distance between the two metals in the series. Farther the metals in the series forming the thermo couple greater is the thermo EMF. And thus the maximum thermo EMF is obtained for Sb-Bi thermocouple.