Abstract Starting from the consideration of a pair of magnetic atoms and of the interaction of a magnetic atom and a coil, the fundamental equation of thermomagnetics is arrived at by realising the work supplied by ponderable forces either in the unordered form of heat, or in a reversible form. The field energy of pairs of atoms belonging both to the sample under consideration is added to the ponderable energy to define its energy, E. If the field energy of the magnetic interaction with external agencies is added, one arrives at the magneto-enthalpy, G. The equations read T d S + d W p o n d = d E + ( J . d H e ) = d G − ( H e . d J )