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It employs two dissimilar metals.
When two such dissimilar metals welded together at once end and subjected to heat, it develops a small DC voltage (EM corresponding to temperature difference between hot junction (measuring junction and cold junction (reference junction).
The temperature-EMF characteristic have an established pattern for different type of Thermocouple elements like T, J, K, N, E, R, S etc.
Temperature - EMF characterstic conform to ANSI MC 96.1 / IEC 584 / DIN 43710 standards.