Demagnetizing Factor

When a specimen or device component of open shape, such as a cylinder, made of a ferromagnetic material is magnetized by an applied field, magnetic poles are formed at the opposite ends of the specimen. These poles cause within the specimen a magnetic field opposite in direction to the applied field. The demagnetizing field H_s of the poles of the specimen is proportional to the specimen’s magnetization J: H_s = NJ. The coefficient N, which is called the demagnetizing factor, is thus equal to the ratio of the strength of the specimen’s demagnetizing field and the specimen’s magnetization. If the specimen is located in an applied magnetic field of strength H_a, then the true strength of the field in the specimen is equal to H_t = H_a – NJ.

The demagnetizing factor can be calculated precisely only for ellipsoids of revolution, which have uniform magnetization. For a sphere, N = ⅓, for a very thin plate, N = 1; and for an infinitely long cylinder in a transverse field, N = ½. For some specimens of simple shape, the demagnetizing factor is calculated by empirical formulas, but in most cases it is determined experimentally.