An electric current of density j flowing in a temperature gradient 1T gives up heat in a reversible way; the amount of heat thus liberated per unit volume and unit time is given by -p.j1T, (1) p. being the Thomson coefficient, which can be shown to be T d r l(Kl )' p.=-e dT(P Ko -~ 5' (2) where ~ is the Fermi energy, and the K's are the usual transport coefficients as defined by Mott and Jones (1936, p. 306, equation (99)), or more generally by Wilson (1953, p. 305).