Citation/Abstract

Between 1820 and 1830, Andre-Marie Ampere presented a controversial new explanation of magnetism as part of a revolutionary theory of a broader domain of phenomena he christened "electrodynamics". The mathematical structure of Ampere's theory was based upon a formula for an electrodynamic force acting between any pair of infinitesimal electric current segments. In a famous 1826 memoir, Ampere gave a very succinct derivation of this formula based upon a well chosen set of four "null experiments".

My dissertation describes the circumstances in which Ampere first recognized the merits of the null experiment technique. Through a study of the manuscripts preserved at the Academie des Sciences, I have discovered that Ampere's breakthrough was brought about by an experiment he performed early in 1822. In fact, there are two respects in which this truly "crucial" null experiment became a significant turning point in Ampere's research. First, it was through a mathematical analysis of this experiment that Ampere discovered the correct expression for one of the factors in his force formula. Hitherto, he had adopted an incorrect version of this factor as a tentative hypothesis. Secondly, the fact that this discovery emerged from an analysis of a null experiment became the inspiration for Ampere's subsequent search for a derivation of his entire formula based solely upon a set of null experiments.

To reveal the rationale of Ampere's reasoning, I devote considerable preliminary attention to the scientific context in which he worked and the relevant details of his career as a scientist and philosopher prior to 1820. I then provide a detailed chronicle of the first eighteen months of his research in electrodynamics, including his debate with a very influential rival theorist, Jean-Baptiste Biot. This period culminated in Ampere's investigation of the "rotary effects" he and Faraday had discovered late in 1821; it was in this setting that Ampere performed the "crucial" 1822 experiment in an attempt to confirm a molecular hypothesis concerning the micro-structure of magnets.