Abstract
The idea that there are two electronic configurations of iron atoms in a face-centred cubic lattice is extended to the iron-nickel alloy system. With a reasonable variation of the energy difference of these two configurations with nickel concentration it is shown that the `invar' effect originates from the thermal excitation of the configuration with lower atomic volume in opposition to the normal anharmonic origin of expansion. The anomalous pressure dependence of the Curie temperature and the variation of Curie temperature with concentration for alloys containing 30-60% Ni are also shown to follow.