The time-decrease of permeability at low magnetizing forces

It has been found that in both normal and incremental permeability measurements the values obtained at low flux densities are increased by the application of a large magnetizing force, such as is involved in the demagnetization of a specimen, this increase then disappearing approximately exponentially with time. This effect, which occurs whether d.c. or a.c. methods of measurement are used, is termed the “time-decrease of permeability.”. It appears to be due to a disturbance of the material from its equilibrium condition by a strong magnetizing force, the time-decrease representing a gradual return to its equilibrium state, which may require several days or even weeks to be effectively completed. A similar disturbance can be produced by severe mechanical treatment.The time-decrease is only considerable below about B = 1000, and within that range varies greatly with the magnetizing force. It also depends enormously on the nature of the material tested, being much greater in highsilicon alloys than in any other type of material. Except at vanishingly small magnetizations, the magnetizing force applied in making the measurements itself exerts a disturbing influence and affects the magnitude of the time-decrease. The effects of a polarizing magnetization and of high temperature have also been examined.Some correlation appears to exist between the magnitude of the time-decrease and the shape of the μ/H curve. but it has not been found possible to establish a quantitative correspondence with any function derived from the μ/H curve. A good mathematical representation of the variation of the permeability with time is given by an expression of the form δ = Kepsilon−Atn, where K, A, and n are constants.The time-decrease is shown to be distinct from ordinary ageing, although the results obtained by Wild and Perrier indicate a close relation between the two phenomena. The practical significance of the effect is discussed in relation to the constancy of inductance of iron-cored coils and to the standardization of the conditions of permeability measurements at low flux densities.