What is the principle behind thermocouple

The correct answer is Seebeck effect.

A thermocouple is a device used to measure temperature. It operates based on thermoelectric principles, specifically the Seebeck effect, discovered by German physicist Thomas Johann Seebeck in 1821.
When two dissimilar electrical conductors (metals) are joined together to form a closed loop, and there is a temperature difference between the two junctions, an electromotive force (emf) or voltage is generated. This phenomenon is known as the Seebeck effect.

The magnitude of the generated voltage is directly proportional to the temperature difference between the hot junction (measuring junction) and the cold junction (reference junction). By measuring this voltage, the temperature at the measuring junction can be determined using calibration data. The relation can be expressed as:
$$V=\alpha (T_{\text{hot}}$$
where $V$ is the generated thermoelectric voltage and $\alpha$ is the Seebeck coefficient (thermoelectric power) of the material pair.

Let's briefly distinguish it from the other options:
1. Peltier effect: The reverse of the Seebeck effect, where passing an electric current through a junction of two dissimilar metals causes heat to be absorbed or released (used in thermoelectric cooling).
2. Thomson effect: Describes the heating or cooling of a single current-carrying conductor when there is a temperature gradient along its length.
3. Joule effect: Refers to the production of heat when an electric current passes through a resistor (heating proportional to the square of the current).