A sectioned spheroid model for cylindrical permanent magnets

A model is proposed for approximating the field strength of permanent magnets. The model has only one adjustable parameter rather than the usual two, and implies a specific pole distribution within the magnet. Calculations and measurements of axial field are compared for six small Alnico magnets of various sizes, with agreement generally within a few percent. The model assumes a distribution of magnetic poles on part of the surface of a uniformly magnetized spheroid, that segment of the surface which lies inside a cylinder of the size of the magnet. The aspect ratio of the spheroid is an adjustable parameter that permits a good fit of calculated and measured data. The calculated internal fields due to the assumed pole distributions support the concept that isolated cylindrical magnets may partially demagnetize near the end faces.