A thermocouple establishes a difference of potential by using what method?

A thermocouple establishes a difference of potential through the method of heating the junction of dissimilar metals. When two different types of metal wires are joined at a junction, and that junction is heated, a voltage is generated due to the thermoelectric effect, specifically the Seebeck effect. This phenomenon occurs because the different metals have varying electron affinities and thermal responses, leading to a voltage differential that can be measured.

The voltage generated is proportional to the temperature difference between the heated junction and the other ends of the wires, which are typically kept at a reference temperature. This characteristic makes thermocouples useful for temperature measurement in various applications, from industrial processes to home heating systems.

The options regarding connecting three similar metals, cooling the wire, and applying a magnetic field do not correctly describe the operational principle of thermocouples. The essence lies in the interaction of dissimilar metals and temperature changes, which is fundamental to the thermoelectric effect utilized by thermocouples.