Langevin , Paul

(1872–1946) French physicist

Langevin, a native Parisian, studied at the Cavendish Laboratory in Cambridge under J.J. Thomson and at the Sorbonne, where he obtained his PhD under Pierre Curie in 1902. He became physics professor at the Collège de France in 1904 and at the Sorbonne in 1909.

Langevin worked on the application of ultrasonic vibrations, which, following Pierre Curie's discovery of piezoelectricity, could be generated by applying a rapidly changing electric potential to a crystal, making it vibrate and produce sound waves in the ultrasonic region. Because ultrasonic wavelengths are shorter than those in the audible range, they are better reflected and Langevin saw that this might be put to military use in World War I. His development of echo location to detect submarines in fact came too late to be used in the war, but this work was the grounding for the later sonar.

Langevin also studied paramagnetism and gave a modern explanation of it incorporating electron theory. In this way he was able to deduce a formula correlating paramagnetism with absolute temperature, which gave a theoretical explanation of the experimental observation that paramagnetic moment changes inversely with temperature. The formula also enabled Langevin to predict the occurrence of paramagetic saturation – a prediction later confirmed experimentally by Heike Kamerlingh-Onnes.

Langevin also studied the properties of ionized gases, and Brownian movement in gases. He publicized in France Einstein's views on the equivalence of energy and mass.