Text: In 1834, Jean Peltier, a French watchmaker and amateur scientist, discovered that the passage of an electric current through the junction between two dissimilar conductors in a certain direction produces a cooling effect. There is a heating effect, that is quite distinct from the more familiar Joule resistance heating effect, when the current is passed in the opposite direction. Peltier hardly realised the true meaning of his observations, let alone the significance that they would have more than a century later as the basis of a new method of refrigeration. Peltier's experiments followed those of Thomas Seebeck, who in 1821 discovered that an electromotive force could be produced by heating a junction between two metals. It is not surprising that the Peltier and Seebeck effects are closely related to one another. What is perhaps more remarkable is the William Thomson (later Lord Kelvin) in 1855 not only derived the relationship between the effects from thermodynamic arguments but, in so doing, he predicted a third thermoelectric effect, then unknown. Thomson obtained the two equations, that are now known as the Kelvin relations and connect the three thermoelectric coefficients, by applying the first and second laws of thermodynamics to a simple thermoelectric circuit, assuming it to be a reversible system. The validity of this approach is questionable (as Thomson well understood) since the thermoelectric phenomena are always accompanied by the irreversible effects of Joule heating and thermal conduction. However, the more reasonable application of the theory of irreversible thermodynamics to this problem also leads to the same relations. The Kelvin relations not only have a sound theoretical basis but they are also well proven experimentally. One of the most convincing practical demonstrations of the validity is the successful use of Harman's method for measuring the thermoelectric figure of merit.

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