An equation of state based upon magnetization scaled to its saturation value, m equals M/Ms, applied magnetic field scaled to its coercive value, h equals H/Hc, distribution of coercivities, and reversible susceptibility Xrev of magnetization and applied stress, scaled to its initial value is proposed for ferromagnetic transducers. Reversible susceptibility divided by the initial susceptibility is the anisotropy function of domain magnetization, decreasing for Terfenol-D nearly linearly with scaled magnetization from one in the demagnetized state to zero at saturation. Measurements of reversible susceptibility, initial, anhysteretic and saturate magnetization curves, and loops for Terfenol-D show that differential magnetic susceptibility is the product of the reversible susceptibility and a cooperative function due to domain interactions. This function is roughly triangular in magnetization having the same slope from each reversal for magnetization magnitude up to half of saturation, at which the onset and decay of cooperation occur. This cooperative function causes parabolic Rayleigh minor loops and sigmoid major B(H) curves truncated by stress demagnetization. Anhysteretic reluctivity increases linearly with the root sum square of shape and stress demagnetizations, the latter added to the anisotropy function. Magnetization loops under stress are modeled and the d* transducer constant dB/ds is derived as a function of stress and magnetization.