Barkhausen Effect II. Determination of the Average Size of the Discontinuites in Magnetization

When the magnetic field-strength acting on a ferromagnetic material is changed, the magnetization changes discontinuously (Barkhausen effect). These discontinuous changes have been examined in 1 mm wires; an expression is derived and experimental arrangements are described for determining their average size for a given material in a given state of magnetization.Experimental determinations of the average size have been made for iron (including a single crystal and a hard-drawn wire), nickel, and several iron-nickel alloys (permalloys). The average size is greatest on or near the steepest part of the hysteresis loop. The greatest average size, expressed as the volume of material the magnetization of which must be changed from saturation in one sense to saturation in the opposite sense to produce the same change in magnetization, is much the same for all of the materials examined, the extremes being 1.2×{}10^-9 cm³ for annealed iron and 45×{}10^-9 cm³ for 50 percent nickel permalloy. This shows that the sizes of the discontinuities do not depend to any considerable extent on the size or kind of crystals.Criticism is made of previous work on the size of the coherence region, the region within which the change in magnetization is confined. Although the effect of a single discontinuity in magnetization may be detected as far as 10 cm from its source because of the eddy-currents induced, the experimental evidence is consistent with the view that the permanent change in magnetization is confined to the volume in terms of which the size of the discontinuity is measured as stated above, always less than 10^-5 cm³.