What does a thermocouple measure?

A thermocouple is a sensor used to measure temperature. It operates on the principle of thermoelectric effect, where two different metals are joined at one end, and when this junction is heated or cooled, it produces a voltage that can be correlated to temperature. Therefore, it effectively measures thermal energy by indicating the temperature difference between the two junctions.

Given this understanding, the choice that best represents what a thermocouple measures is thermal energy. It is important to distinguish that while a thermocouple generates an electrical signal, it is fundamentally used to gauge thermal conditions, thus providing a direct measurement of temperature changes rather than measuring energy directly in forms such as electrical, mechanical, or kinetic.