Ampère was present at the Académie des Sciences on Sept. 11, 1820, when François Arago performed - for the first time in France - Hans Christian Oersted’s experiment demonstrating the magnetic effects of current-carrying wires on magnetized needles. Inspired by Oersted’s discovery, Ampère immediately concluded that magnetism was electricity in motion, an intuitive leap which he sought to confirm by experiment.

During September and October 1820, Ampère per-formed a series of experiments designed to elucidate the exact nature of the relationship between electric current-flow and magnetism, as well as the relationships governing the behavior of electric currents in various types of conductors. His investigations, reported weekly before the Académie des Sciences, established the new science of electrodynamics.

Ampère’s most detailed report on the events of September and October 1820 was published as a lengthy two-part memoir in the Annales de Chimie et de Physique. Written hurriedly and in disjointed segments, it is a rich source of information in spite of its chronological errors. . . .” (Hofmann, p. 238). Among the discoveries described in this memoir are Ampère’s demonstration of the tangential orientation of a magnetic needle by an electric current when terrestrial magnetism is neutralized; his proof that conducting planar spirals attract and repel each other and respond to bar magnets in an analogy to magnetic poles; and his demonstration of electrodynamic forces between linear conducting wires. The memoir’s plates illustrate the several instruments that Ampère devised to carry out his experiments (see below).

Ampère’s scientific genius, while capable of remarkable leaps of insight, was somewhat lacking in organization and discipline. It often happened that Ampère would publish a paper one week, only to find the following week that he had thought of several new ideas that he felt ought to be incorporated into the paper. Since he could not alter the original, he would add his revisions to the separately published reprints of the paper, and even modify the revised versions later if he felt it necessary; some of his papers exist in as many as five different versions.

A separate reprint of Ampère’s Mémoire was issued in 1821; however, it differs substantially from the journal publication, which must be considered the original version of this foundation document in electrodynamics.

DSB. Hofmann, Andre-Marie Ampère, ch. 7 (containing a detailed account of Ampère’s investigations). Norman 43 (1821 reprint). 37292
 

Illustrations of Ampère's apparatus from Mémoire présenté à l’Académie royale des sciences: