Q.What is the Thomson effect?

The general idea. The thermoelectric effect refers to the reversible, two-way conversion between a temperature difference and an electrical voltage: applying a temperature difference across a suitable device generates a voltage, and conversely, driving a current through such a device can itself generate a temperature difference. Three distinct, historically named manifestations of this idea appear in this unit.

The Seebeck effect. In a closed circuit made from two DISSIMILAR metals joined at two junctions (a "thermocouple"), maintaining the two junctions at different temperatures generates an emf that drives a "thermoelectric current" around the loop. This emf's size depends on (i) which particular pair of metals is used, and (ii) the temperature difference between the junctions; reversing which junction is hot and which is cold reverses the current's direction, so the Seebeck effect is fully reversible. Its applications include thermoelectric (Seebeck) generators that convert waste heat into electricity in power plants, automotive thermoelectric generators that recover exhaust heat to improve fuel efficiency, and thermocouples/thermopiles used as accurate temperature-measuring instruments.

The Peltier effect. Peltier (1834) discovered the OPPOSITE-direction phenomenon: passing a current THROUGH a thermocouple circuit causes heat to be evolved at one junction and absorbed at the other. In a copper-iron thermocouple, current flowing from copper to iron at one junction absorbs heat there (cooling it), while current flowing from iron to copper at the other junction liberates heat there (heating it); reversing the current's direction swaps which junction gets hot and which gets cold, so the Peltier effect, like the Seebeck effect, is fully reversible. …