Which device uses two different metals welded together to produce a potential difference?

The device that uses two different metals welded together to produce a potential difference is the thermocouple. A thermocouple operates based on the Seebeck effect, which states that when two dissimilar metals are joined at two junctions and are subjected to different temperatures, a voltage is generated between these junctions. This voltage is a direct result of the temperature differential and is used to measure temperature or generate power.

In contrast, a diode is a semiconductor device that allows current to flow in one direction and is not based on the principles of welding metals together. A resistor is a passive electrical component that opposes the flow of current, primarily used to control voltage and current levels in a circuit. A capacitor stores electrical energy in an electric field and is not involved in generating a potential difference from the combination of metal materials.

Thus, the thermocouple uniquely fulfills the criteria of generating a potential difference through the junction of two different metals.