Perrin, Jean Baptiste (1870–1942)

French physicist who produced the crucial evidence that finally established the atomic nature of matter. Assuming the atomic hypothesis, Perrin demonstrated how the phenomenon of Brownian motion could be used to derive precise values for Avogadro's number. He was awarded the Nobel Prize for Physics in 1926 for his confirmation of the discontinuous structure of matter.

Perrin also contributed to the discovery that cathode rays are electrons. His experiments included imposing a negative electric charge on a fluorescent screen onto which various rays were focused. As the negative charge was increased, the intensity of fluorescence fell.

Perrin was born in Lille and studied in Paris at the Ecole Normale Supérieure. He worked at the Sorbonne from 1897, becoming professor in 1910, but in 1940, during World War II, his outspoken antifascism caused him to flee the German occupation. He went to New York.

His book Les Atomes/Atoms (1913) describes his Nobel prize-winning study of Brownian motion.