The Action of a High-Frequency Alternating Magnetic Field on Suspended Metallic Rings and Disks

An experimental study of the torque on suspended nmetallic rings and solid disks caused by a high-frequency alternating magnetic field indicates (1) that G. W. Pierce´s expression for the torque on a ring is valid over a wide range of frequencies, and (2) that, as a function of frequency, intensity of field, thickness, and conductivity, the torque on a disk is sufficiently similar to that for a ring to suggest that it varies as τp²Hv²/R(1+τ²p²) in which τ is the time constant of the disk, R, its effective resistance, and p is 2π times the frequency of the applied magnetic field whose virtual intensity is Hv. Measurement of this torque on a ring or disk apparently constitutes a convenient and practicable method of measuring the intensity of a high-frequency magnetic field. Comparative measurements on a ring and disk indicate that the greater part of the induced circular current in a disk at high frequencies flows near its periphery, primarily because of the shielding action of the peripheral currents, a result in agreement with experimtents made previously by Zenneck. A reproducible depression in the torque-frequency curve of a thin silver disk was observed at 700 kilocycles.