Langevin's ultrasonic capacitive transducer (1915–1917)

This historical review traces the genesis, design, and performance of the first underwater source of a beam of ultrasound, created in 1915 by the French physicist Paul Langevin, using a capacitive transducer. The idea was derived from a proposal by the Russian inventor Constantin Chilowsky for directional underwater detection and communication, “On the possibility of vision under water” [Klyukin and Shishkov, Konstantin Vasilievich Shilovski 1880–1958, https://www.klex.ru/1xja]. The historical significance of this first successful electromechanical ultrasonic emitter, operating at 100 kHz, has been obscured by the much greater impact from Langevin's discovery of quartz piezoelectric ultrasonic transduction in 1917. In reality, as Langevin noted, it was “this electrostatic solution which …. gave proof of the possibility of emitting ultra-sonic waves” [P. Langevin, Hydrographic Rev. 2(1), 53–91 (1924)]. Langevin was later doubtful that capacitive transducers could ever compete with piezoelectric ones. He recognised the inherent limitations on the maximum power that this method could generate, the required voltage gradient being at the boundary of electrical breakdown.