André-Marie Ampère

André-Marie Ampère was born in Lyon in 1775 to a prosperous silk merchant who refused to send his son to school and instead let him roam freely through the family library. By thirteen, Ampère had read every one of the twenty-eight volumes of Diderot and d'Alembert's Encyclopédie. He learned Latin so he could read Euler and Bernoulli in the original, taught himself the calculus from Lagrange's textbook, and was reconstructing the orbit of Halley's comet from first principles before he was sixteen. The Revolution shattered this idyll: in November 1793 his father, a mild royalist who had served briefly as a justice of the peace, was guillotined in the Lyon Terror. Ampère collapsed into a year of stunned grief from which, by his own account, he was only rescued by reading the lyric poetry of Antoine Roucher and the botanical treatises of Rousseau.

He recovered, married Julie Carron in 1799, and worked his way up through provincial teaching posts to a chair of mathematics at the École polytechnique in Paris by 1809. Then on 11 September 1820, François Arago demonstrated Ørsted's experiment at a meeting of the Académie des sciences. Ampère had never given electromagnetism a serious thought, but the result struck him with the force of a revelation. Within a week he was experimenting; within two months he had derived the force law between two parallel current-carrying wires (currents in the same direction attract, opposite repel, force scales as 1/r); within a year he had formulated what we now call Ampère's law (the line integral of B around a closed loop equals μ₀ times the enclosed current); and he had coined the words "electrodynamics" and "electric current" — neither of which had existed in 1819. He worked tightly with Biot, who was simultaneously developing his own (more empirical) approach with Savart, but where Biot stayed close to the experimental data, Ampère insisted on a unified mathematical framework. His 1827 Théorie mathématique des phénomènes électrodynamiques uniquement déduite de l'expérience is the founding text of mathematical electromagnetism.

Personally, Ampère was a wreck for most of his life. Julie died in 1803 after four years of marriage; his second marriage to Jeanne Potot in 1806 was so unhappy that they separated within a year. He suffered chronic depression, financial difficulties, and bouts of religious crisis. He died of pneumonia in Marseille in 1836 on a school inspection trip, aged sixty-one, and was buried in the local cemetery before his family could be informed. The SI unit of electric current carries his name. Maxwell, who came after, wrote that "the experimental investigation by which Ampère established the law of the mechanical action between electric currents is one of the most brilliant achievements in science. The whole, theory and experiment, seems as if it had leaped, full grown and full armed, from the brain of the Newton of electricity."