Paul-Jacques Curie

…in 1880 by Pierre and Paul-Jacques Curie, who found that when they compressed certain types of crystals including quartz, tourmaline, and Rochelle salt, along certain axes, a voltage was produced on the surface of the crystal. The next year, they observed the converse effect, the elongation of such crystals upon…

…Pierre Curie and his brother Jacques in 1880. Applications of ultrasonics, however, were not possible until the development in the early 20th century of the electronic oscillator and amplifier, which were used to drive the piezoelectric element.