Before World War I, Cady investigated arc discharges and radio detectors, but during the war became interested in crystals as he worked with General Electric Company's Research Laboratory, Columbia University, and the Naval Experimental Station in New London, Connecticut, on using high-frequency sound generated by piezoelectricity to detect submarines. His early experiments employed Rochelle salt crystals as transducers. After noticing that a quartz crystal connected to a variable-frequency electronic oscillator would vibrate strongly at a very specific frequency, but that at other frequencies it would not vibrate at all, he had the insight to apply crystal oscillators to radio frequency applications. The papers consist exclusively of letters (correspondence) addressed to Hazeltine, ac.
