The Barkhausen Effect

Abstract

Barkhausen effect for silicon steel.---In a typical experiment a strip of silicon (4.2%) steel, after having been carried through several hysteresis cycles and brought to a steep part of the $B\ensuremath{-}H$ curve, was subjected to a magnetic field which was increased in a continuous manner by 0.13 gauss in 2 sec. To obtain a record of the discontinuities in magnetization, the specimen was surrounded by a small search coil connected through an amplifier to a moving coil oscillograph. The oscillograph records show many almost instantaneous deflections with a random distribution both as to time of occurrence and as to magnitude, each followed by an approximately exponential return to zero. The duration of an impulse depends probably on eddy currents in the specimen. A rough time constant of 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ sec. is computed for a 4% silicon steel wire, 1 mm in diameter. The apparatus was calibrated by means of artificial impulses of the same type. Assuming each impulse is due to a sudden saturation of a small portion of the material, the change in magnetic moment of this portion is found to vary from.001 to.008 e.m.u.; the average change.003 is sufficient to saturate a volume of 1.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ cc. This volume, while comparable with that of a crystal grain in the particular case described, was found not to depend upon grain size in any marked degree in other experiments. The results favor the suggestion of Barkhausen that magnetic materials magnetize discontinuously, but leave open the question as to what determines the size and shape of the portions which suddenly change.