What is the primary principle of operation for a thermocouple?

The primary principle of operation for a thermocouple is based on the Seebeck effect. This phenomenon occurs when two dissimilar metals are joined at two junctions, and there is a temperature difference between those junctions. The Seebeck effect generates a voltage that is proportional to the temperature difference.

When one junction is heated, the metal atoms at that junction become more energetic and produce a measurable voltage across the circuit, which is indicative of the temperature difference. This voltage can be interpreted to determine the temperature at the heated junction, making thermocouples widely used for temperature measurement in various applications due to their simplicity and reliability.

In contrast, the other effects mentioned have different applications: the Joule heating effect involves heat generation in a conductor when an electric current passes through it, the Hall effect relates to the generation of a voltage in a conductor subjected to a magnetic field, and the piezoelectric effect pertains to the generation of electric charge in certain materials when they are mechanically stressed.