Contribution d'Ampère au « théorème d'Ampère ».

SUMMARY. — The author has aimed at showing that the theorem which is known as Ampere's Theorem, an admirable expression of his deepest thought, cannot have been stated by Ampère himself. As can be established by an unpublished document in the archives of the Académie des Sciences, it is Ampere's physical theory of magnetism itself which blocked his researches, and not difficulties of a mathematical nature. This article contains a comparison between Ampere's procedure in the unpublished document in the archives and Maxwell's in his Treatise on Electricity and Magnetism, concerning the problem of the action of an indefinite vertical current on a magnet pole.