Examinations of ferromagnetic behaviour under conditions likely on power systems show that eddy currents may be allowed for by including an appropriate linear resistor in mathematical models. They also show that hysteresis paths depend on the previous history of a core but are hardly affected by the time derivative of the flux. Current-transformer studies are described to illustrate a computational method in which discrete points on a number of hysteresis paths are specified. An extrapolation technique is used, and consideration is given to points at which flux reversals occur. It is shown that highly-accurate representation may be achieved.