Equations of state along the road to Arrott's last plot

A modification of the Weiss theory of ferromagnetism is used to create an approximate equation of state that relates the magnetization, field and temperature. The high-temperature susceptibility, the critical region and the temperature dependence of the spontaneous magnetization to low temperature are found by making an approximation for the inverse Brillouin function and then adding critical exponents in the proper places. The coefficient of the Weiss effective field is treated as dependent on the magnetization, introducing an additional parameter. This equation of state is applied to results from studies of iron whiskers.