What is an example of thermal action that can establish a difference of potential?

The phenomenon described in option A, heating the junction of two dissimilar metals, is known as the thermoelectric effect or Seebeck effect. When two different metals are joined together and their junction is heated, a temperature difference creates a voltage difference due to the different electron mobility in the two metals. This results in a flow of charges, establishing a difference of potential across the junction.

This principle is commonly utilized in thermocouples, which are devices used to measure temperature by converting thermal energy into electrical energy. The unique property of different metals to generate varying voltages at different temperatures is the key to understanding how thermal action can produce a potential difference.

Other options do not directly illustrate thermal action leading to a difference of potential. For example, passing current through a conductor relates to electrical current flow rather than thermal action, breaking a circuit path interrupts the flow of electricity without directly creating a potential difference, and increasing the voltage in a circuit is a matter of electrical magnitude rather than thermal influence.