Thermocouple
Click for Zoom

It employs two dissimilar metals.
IWhen 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).
IThe temperature-EMF characteristic have an established pattern for different type of Thermocouple elements like T, J, K, N, E, R, S etc.
ITemperature - EMF characterstic conform to ANSI MC 96.1 / IEC 584 / DIN 43710 standards.