Incoming correspondence, 1896-1951, including numerous letters from French and foreign scientists such as: Georges Darmois, Gustave Ribaud, Franȯis Bruhat, Jean Cabannes, Andr ̌Debierne, Fric, Gabriel Fox︠, Ernest Esclangon, Franȯis Croze, Pierre Sv̈e, Henri Buisson, Jules Baillaud, John-Paul Mathieu, Paul Langevin, Marie Curie, Pierre Weiss, Pieter Zeeman, and Walter Ritz, as well as Cotton's students Azinir̈es, R. Fortrat, Olivier, and Soury. Also incoming correspondence from his physics teacher Joseph-Denis Izarn, 1842-1925; laboratory notebooks, 1895-1920; notebooks on physics related to the war of 1914-1918; tests and adjustments of the fluxmeter, of the Cotton-Weiss sound-ranging system; documents concerning the adjustment, the installation, and the accounts of the large electro-magnet of Bellevue; military research, 1939-1940; laboratory notes, manuscripts of articles, articles by himself and by his students, biographical notes written after his death.