Ultrasonics in Fluids

Abstract

DURING the recent meeting of the British Association at Newcastle upon Tyne, Section A (Mathematics and Physics) held a symposium on ultrasonics in fluids. Most of the papers read at this symposium, concerned applications of the ultrasonic interferometer, which may be regarded as a modern development of the familiar Kundt's tube into a precision instrument for studying those properties of gases and liquids which come into play when the molecules are set into vibration as a whole. In essence, the instrument consists of a quartz crystal in piezo-electric vibration, sending out plane waves into the fluid which are reflected back to the crystal, there to react upon the circuit which maintains the piezo-electric oscillations. The reflector usually consists of a plate mounted on a micrometer screw which is carried parallel to itself in a direction perpendicular to the ultrasonic wave-front, thereby recalling several types of optical interferometer. In the original method of Pierce, the magnitude of the resonance, as indicated by the maintaining circuit, at each half wave-length shift of the screw is observed through the change in the anode current of the circuit. With some precautions, the magnitudes of these current changes may be used to calculate the absorption coefficient of the radiation in the fluid, and, of course, the screw settings for these resonances enable the velocity in the fluid to be calculated if the frequency of the crystal oscillations can be accurately measured.