. Biophysical research methods. Biophysics -- Research. X. ULTRASONIC VIBRATIONS 315 material. The actual relative change in length obtained in operation is determined not only by the material used but also by the method of clamping the rod and by the acoustic load presented to the rod. Ex- amination of the graph shows that for nickel the addition of a pre- magnetizing field on the order of 50 oersteds will not only prevent frequency doubling, but will also place the operating point of the rod at a position where the relative change in length is faii'ly large for a given change in magnetic fie
. Biophysical research methods. Biophysics -- Research. X. ULTRASONIC VIBRATIONS 315 material. The actual relative change in length obtained in operation is determined not only by the material used but also by the method of clamping the rod and by the acoustic load presented to the rod. Ex- amination of the graph shows that for nickel the addition of a pre- magnetizing field on the order of 50 oersteds will not only prevent frequency doubling, but will also place the operating point of the rod at a position where the relative change in length is faii'ly large for a given change in magnetic field. This point is rather critical Fig. 4. Relative change in length as function of magnetic field strength for various magnetostric- tive materials. Negative values signify contraction; positive values denote expansion with increasing magnetic 200 400 MAGNETIC FIELD (W), oersteds 600 for best operation of the rod. As a general rule, the rods are not premagnetized to this point but rather placed in a special magnetic yoke that will allow the superposition of both and fields. It is important to note that the peak field cannot exceed the bias- ing magnetic field without producing excessive distortion of the sound emitted by the rod. Consider as a simple example a nickel rod cm. in length vibrating at 20 kilocycles with a biasing field of 50 oersteds. If we assume that the superposed field has an amplitude of 10 oersteds, this will produce, as seen in Figure 4, an amplitude of vibration of the free bar of: A = (5 X 10-6)() = X lO"" Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Uber, Fred Murray, 1905-. New York : Interscience Publishers
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