  Onsager's reciprocity relations for the coefficients of transport equations
are now 87 years old. Sometimes these relations are called the Fourth Law of
Thermodynamics. Among others they provide an effective criterion for the
existence of local equilibrium and of microscopic reversibility. Since the
beginning of the century Onsager's relations have seen a revival in the field
of spincaloritronics. There the relations are very helpful in judging the
utility of modern devices for electronic data processing.
