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In page Crystal oscillator:
"Piezoelectricity was discovered by Jacques and Pierre Curie in 1880. Paul Langevin first investigated quartz resonators for use in sonar during World War I. The first crystal-controlled oscillator, using a crystal of Rochelle salt, was built in 1917 and patented[1] in 1918 by Alexander M. Nicolson at Western Electric, although his priority was disputed by Walter Guyton Cady.[2] Cady built the first quartz crystal oscillator in 1921.[3] Other early innovators in quartz crystal oscillators include G. W. Pierce and Louis Essen.[citation needed]
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