Ultrasound

Ultrasound, mechanical vibrations having a frequency lying above the limit of audibility of sound (20,000 vibrations/sec.). They can be obtained by means of piezoelectric quartz, connected to a generator of electromagnetic oscillations. The reason for their occurrence lies in the periodic compressions and expansions of quartz when a periodically changing difference of potential is applied to it. In the presence of resonance (see) between the frequency of electromagnetic oscillations and the natural frequency of quartz oscillations, the amplitude of ultrasonic oscillations sharply increases. If quartz is placed in a vessel with oil, then the pressure created by U. can be made so large that a fountain several centimeters high begins to beat from the surface of the oil above the quartz. Under the action of U., the most diverse emulsions can be easily obtained, just as by means of them one can also affect the course of certain chemical reactions. In the transmission of ultrasonic oscillations into solid bodies, it is possible, by studying their propagation in bodies, to detect the existence of internal defects (cavities, heterogeneities, etc.). Ultrasound is used for underwater signaling. Langevin (1917), who first constructed a powerful signaling device, noticed that fish entering the beam of ultrasonic waves perish. According to Wood's observations, U. cause the death of small organisms, fish and frogs, the rupture of blood corpuscles, etc. Their effect on the human body is accompanied by the release of heat; this allows one to think of the possibility of using U. for therapeutic purposes, although to this day U. have not yet found application in medicine. Ultrasound can also be obtained by means of ferromagnetic rods, which come into magnetostrictive oscillations during their periodic remagnetization.