Which effect describes the principle on which thermocouples operate?

The principle on which thermocouples operate is known as the Peltier-Seebeck effect. This effect arises from the interaction between temperature differences and electric potential in a circuit made of two different metals. When there is a temperature difference between the junctions of these two metals, it creates a voltage, allowing thermocouples to convert thermal energy into electrical energy. This phenomenon is fundamental to the functioning of thermocouples, which are widely used for temperature measurement.

In contrast, frictional heating pertains to heat generated due to the friction between surfaces in motion, which is not relevant to the operation of thermocouples. Thermal conductivity deals with the ability of a material to conduct heat, and while it's an essential property in various thermal systems, it does not explain how thermocouples work. Caloric theory, an outdated concept, suggests that heat is a fluid-like substance called caloric, which is not applicable in explaining the principles of thermoelectric devices like thermocouples. Thus, the Peltier-Seebeck effect directly describes the mechanism behind thermocouples, making it the correct choice.