What does a thermocouple rely on for its function?

A thermocouple operates based on the principle of thermoelectricity, which involves the generation of a voltage difference that corresponds to temperature differences at its junctions. When two dissimilar metals are joined at two points, and those points are subjected to different temperatures, a voltage is generated due to the Seebeck effect. This voltage difference is directly related to the temperature difference between the hot junction (where the temperature is measured) and the cold junction (usually at a reference temperature). Therefore, the proper functioning of a thermocouple relies fundamentally on this difference in voltage generated by thermal gradients.

The other options, while related to various physical phenomena, do not describe the fundamental operational principle of a thermocouple. Change in resistance pertains more to resistance thermometers (RTDs), heat flow does not specifically denote the mechanism of voltage generation in this context, and pressure difference relates to fluid dynamics and thermodynamic processes, which are not the basis for how a thermocouple measures temperature.