Thermoelectric Effect - Thomson Effect

The Thomson effect was predicted and subsequently observed by Lord Kelvin in 1851. It describes the heating or cooling of a current-carrying conductor with a temperature gradient.

Any current-carrying conductor (except for a superconductor) with a temperature difference between two points either absorbs or emits heat, depending on the material. If a current density J is passed through a homogeneous conductor, the heat production q per unit volume is:

where ρ is the resistivity of the material, dT/dx is the temperature gradient along the wire and μ is the Thomson coefficient. The first term is the Joule heating, which does not change in sign; the second term is the Thomson heating, which follows J changing sign.

In metals such as zinc and copper, whose temperature is directly proportional to their potential, when current moves from the hotter end to the colder end, there is a generation of heat and the positive Thomson effect occurs. Conversely, in metals such as cobalt, nickel, and iron, whose temperature is inversely proportional to their potential, when current moves from the hotter end to the colder end, there is an absorption of heat and the negative Thomson effect occurs.

If the Thomson coefficient of a material is measured over a wide temperature range, it can be integrated using the Thomson relations to determine the absolute values for the Peltier and Seebeck coefficients. This needs to be done only for one material, since the other values can be determined by measuring pairwise Seebeck coefficients in thermocouples containing the reference material and then adding back the absolute thermopower of the reference material.

Lead is commonly stated to have a Thomson coefficient of zero; in fact, it is non-zero, albeit being very small. In contrast, the thermoelectric coefficients of all known superconductors are zero.