A Dynamic Study of Magnetostriction

In a recent paper by Professor G. W. Pierce1 are described various effects which occur when a magnetic rod has superimposed upon it an alternating axial magnetic field of a frequency in the neighborhood of the natural mechanical frequency of the rod. Rods under the influence of magnetostriction can be made to play a part somewhat analogous to quartz crystals under the influence of piezoelectricity. If a ferromagnetic rod or tube is clamped so that it lies along the axis of a coil, the combination becomes an electrical vibration instru ment, and may be treated in the same way that telephone receivers are treated by Kennelly and Pierce,2 or by Kennelly in his book on vibration instruments.3 The present paper describes the measure ment of the dynamic electrical characteristics of various magnetostric tive rods and tubes, also the magnitude of vibration and other measurements that could be made conveniently at the same time. Method of Measurement.?The motional resistance and reactance are measured by connecting the coil containing the rod into an alternating current bridge, as in the reference2 mentioned above. In this particular experiment, the bridge was designed to pass a direct current as well as the alternating current through the coil, thus it was possible to maintain a known field on the rod at any time. The coil is a long solenoid to insure a uniform magnetic field. The coil is 120 cm. long, has an inside diameter of two inches, and at its center developes a field of 32.5 gauss per ampere. The rods are all about 90 cm. long, and their natural frequencies lie between 2300 and 3000 cycles per second. In all measurements the superimposed alternating field is small compared to the direct or polarizing field. In measurement of amplitude of vibration, an extremely light phosphor bronze strip upon which is mounted a small mirror is fastened from the end of the rod to the stationary frame to which the center of the rod is clamped. The longitudinal vibration of the rod thus gives an angular motion of the mirror. A point source of light reflected from the mirror is observed in a telescope with a