Between 1915 and 1918, Paul Langevin demonstrates the feasibility of using piezoelectric quartz crystals to both transmit and receive ultrasound pulses, detecting submerged submarines at ranges up to 1300 metres. The system, later called sonar, experimentally validates Constantin Chilowsky's 1915 theoretical proposal. Langevin and Chilowsky jointly patent the invention in the US in 1916 and 1917. The technology was used successfully in World War II and gave rise to other applications such as depth sounding and medical ultrasound.