The Inner, Initial, Magnetic Permeability of Iron and Nickel at Ultra-High Radiofrequencies

Abstract

A new method for the measurement of the ultra-high frequency, initial, inner permeability of ferromagnetic materials has been developed which gives much greater accuracy and is much faster than the older methods. The probable error is under one percent at a frequency of 197 megacycles per second, as contrasted with previous methods in which the error is of the order of 10 to 20 percent. This improvement is due to three factors: (1) the use of a coaxial cable instead of Lecher wires, (2) the use of a fine central wire and (3) correction for the influence of the finite conductivity of the resonance system. The magnetic permeability of high purity, commercial iron wire was found to be 53.8±0.3 at 1.97×${10}^8$ cycles per second, and 25°C. It was constant for all values of tension up to the breaking point of the wire at 980 grams force. It also remained constant for superimposed longitudinal magnetic fields up to 100 oersteds. The permeability of commercially pure nickel was found to be 3.61±0.08 at the same frequency and temperature. As the temperature was raised the permeability increased to a value of 12.22 at 320°C, beyond which it dropped rapidly to unity at 370°C.