ANS:B - Seeback effect

Explanation:-

in 1981 seebak found 2 different metals having different temperature junctions that deflected a compass magnet based on ampere law. thermocouple can measure temp diff and to measure it need a reference temp to compare with standard readings which in general given by

V = A(Th-Tc).

V-Voltage difference between two dissimilar metals.
A- Seebeck coefficient.
Th - Tc - Temperature difference between hot and cold junctions respectively.