The paper summarizes the development of high-permeability alloys from early researches to the commercial products of the present day.The difficulties encountered during manufacture, and the precautions necessary in these metallurgical processes, are described.Stress is laid on the correct methods of application to ensure that the magnetic properties of the alloys are utilized to full advantage, and the need for heat treatment is emphasized . Propositions are put forward to explain the mechanism of this heat-treatment process, and the practical procedure is given.Data on curves show the electrical and magnetic properties of the alloys.The paper finally discusses the practical applications of high-permeability materials, giving examples of commercial instruments and indicating cases in which their extended use would be advantageous.