The permeability of ferromagnetic materials at frequencies between 105 and 1010 c/s

Methods of measuring the permeability of ferromagnetic materials in high-frequency fields are described, and the main results obtained by various investigators are collated and discussed. It appears that hysteresis effects cease between 105 and 107 c/s, and that the permeability decreases with increase of frequency until it reaches unity in the region of 1011 c/s. Various explanatory hypotheses are discussed. The most probable explanation is that the phenomena are due to eddy currents which delay the magnetization processes. These eddy currents are caused by the extensions or rotations of the magnetic domains, and act so as to slow down and finally prevent these changes. The theoretical data evaluated on the basis of this hypothesis are in general agreement with the practical observations.