In 1918-1919 Walter G. Cady was the first to recognize the significant electrical consequences of the fact that piezoelectric crystals resonate at very sharp, precise and stable frequencies. Cady was also the first to suggest the employment of these properties, first as frequency standards and then to control frequencies of electric circuits-an essential component in electronic technology. Cady's discovery originated in the course of research on piezoelectric ultrasonic devices for submarine detection (sonar) during World War I. However, for the discovery Cady had to change his research programme to crystal resonance. This change followed Cady's experimental findings and the scientific curiosity that they raised, and was helped by the termination of the war. Cady's transition was also a move from "applied" research, aimed at improving a specific technology, to "pure" research lacking a clear practical aim. This article examines how Cady reached the discovery and his early ideas for its use. It shows that the discovery was not an instantaneous but a gradual achievement. It further suggests that disinterested "scientific" research (rather than "engineering" research) was needed in this process, while research aimed at design was required for the subsequent development of technological devices.