216 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



after, two unrelated scientific discoveries were combined by Langevin 

 to provide for the first time a dependable source of ultrasonic waves 

 of controllable frequency and intensity. To replace the inherently 

 unstable Poulsen arc, Langevin chose the newly developed and far 

 more stable vacuum-tube oscillator. To replace the mica condenser 

 he chose a piezoelectric crystal. 



Previously, the piezoelectric effect had had no practical application. 

 A French apothecary, Pierre de la Seignette, of La Rochelle, in 1672 

 discovered the crystal known as Rochelle salt. In 1880, Pierre and 

 Jacques Curie found that mechanical stresses produced electric charges 

 on the faces of a Rochelle-salt crystal. The inverse of this piezoelec- 

 tric (pressure electricity) effect was theoretically predicted by Lipp- 

 mann in 1881 and experimentally verified by the Curies the following 

 year — a voltage applied across the crystal produced a change in the 

 thickness of the crystal. Although it was later discovered that many 

 other crystals, including quartz, had this same property, the piezo- 

 electric effect remained nothing more than a scientific curiosity until 

 1917. In that year Langevin, who became acquainted with the effect 

 while a student in the laboratory of the Curie brothers, applied the 

 output of a vacuum-tube oscillator across a quartz crystal to produce 

 the first stable, powerful generator of ultrasonic waves. 



Today Langevin's generator, with relatively minor improvements, 

 remains the best source of ultrasonic radiation of precisely controllable 

 frequency and intensity. The high-frequency voltage output of a 

 vacuum-tube oscillator is applied to electrodes on opposite faces of 

 a properly cut crystal. When the oscillator frequency is adjusted 

 to the natural resonant frequency of the crystal, powerful mechanical 

 vibrations result, and a beam of ultrasonic waves is radiated through 

 the medium surrounding the crystal. In addition to quartz and 

 Rochelle salt, a number of other natural and synthetic crystals may be 

 employed to serve particular applications. 



LATER GENERATORS 



During the decade following the first World War, progress in ultra- 

 sonics again slowed to a snail's pace except for certain classified mili- 

 tary developments in underwater signaling and the development of 

 the magnetostriction oscillator in 1925 by G. W. Pierce. Pierce used 

 a ferrous-metal rod as the core of a solenoid which was energized by 

 an alternating current. As the result of magnetostriction, the fer- 

 rous rod periodically changed its length in the alternating magnetic 

 field and a beam of ultrasonic waves was radiated from the end of 

 the vibrating rod. This magnetostriction generator is widely used 

 today, but is limited in the range of frequencies it can generate. For 

 high frequencies the length of the ferrous rod required for resonance 



