The paper outlines the theory, method of construction and use of a family of curves allowing a rapid and accurate estimation of the average m.m.f. expended on tapered magnetic teeth. The parameters used result in less complicated relationships between the variables than with any alternative accurate method; this somewhat simplifies the calculations necessary to obtain the curves for magnetic materials having differentB/Hcharacteristics. An analysis is made of the inaccuracy of the simple “one-third” rule under varying conditions of magnetic loading and tooth profile, and it is shown that the use of this rule can lead to very considerable errors.