Magnetic Permeability of Iron and Magnetite in High Frequency Alternating Fields

Abstract

Relative values of the permeability of cast-iron filings, iron wires, and iron powder in high frequency magnetic fields.---Wwedensky and Theodortschik have found the magnetic permeability of iron, steel, and nickel in alternating fields to be abnormally large in certain frequency bands (at about 100 meters for iron) and nearly normal in other regions. The general appearance of the phenomenon suggested the existence, in the material, of resonators corresponding to these frequencies. The phenomenon has been observed also by Kralovec. Two experimental methods have been followed in the present investigation, one the resonance method and the other the heterodyne method. Both utilized, in principle, the measurement of the change in inductance of a coil due to the introduction of the sample of material into it. The wave-length ranges covered were from 80 to 1700 meters by the heterodyne method and from 50 to 160 by the resonance method. The heterodyne method was used in an improved form which eliminated drifts. The results are in disagreement with those of Wwedensky and Theordortschik and with those of Kralovec. No anomalous change in permeability was found at any frequency. The following errors, which may have misled previous investigators, were found in the course of the present work: (a) anomalous behavior of capacities in series when these are connected in tube circuits, an effect the nature of which is unknown but which is related in some way to the length of the connecting wires; (b) a general variation in the apparent permeability as measured by one coil, presumably due to effects of distributed capacity; (c) apparent anomalies when a number of coils are used without comparing the results at the same wave-length; (d) an apparent anomaly in the permeability, at a critical frequency, arising from the presence of a metal shield inside the coil; (e) an effect of drift in the heterodyne method.The absolute value of the permeability of powdered magnetite.---The permeability of magnetite in powdered form has been measured by two fairly independent methods. The values decrease from about 1.532 at 132.2 meters to 1.401 at 85.8 meters. After due allowance is made for density of packing, these values compare favorably with the results obtained by a static method by Welo and Baudisch.