Jean-Baptiste Biot

Jean-Baptiste Biot was born in Paris in 1774, the son of a treasury clerk who pushed him toward a commercial career. He drifted instead into the artillery during the Revolutionary wars, was briefly imprisoned as a royalist sympathiser in 1794, then talked his way into the École polytechnique on its first day of classes in late 1794. He impressed Lagrange and Laplace, who took him on as protégés; by twenty-six he held the chair of mathematical physics at the Collège de France, a post he kept for the next sixty-two years. In 1804 he made the first scientific balloon ascent in history, rising over four kilometres above Paris with Joseph Louis Gay-Lussac to measure the variation of terrestrial magnetism with altitude — a flight more notable for what it ruled out (no measurable variation up to 4,000 m) than for what it discovered.

By 1820 Biot was the senior physicist of the Académie des sciences, established for his work on the polarisation of light in the atmosphere and on the refractive properties of crystals. When Arago demonstrated Ørsted's experiment to the Académie that September, Biot immediately sat down with the young military surgeon Félix Savart, then visiting Paris, to work out a mathematical description of the force exerted on a magnetic pole by an arbitrary segment of current-carrying wire. They presented their result on 30 October 1820, three weeks before Ampère's announcement: the magnetic field at a point produced by a small element dℓ of current I is dB ∝ I dℓ × r̂ / r² — the inverse-square structure of Coulomb's law, with a vector cross-product that captures the rotational geometry Ørsted had observed. The Biot–Savart law, in modern notation with μ₀/4π out front, is still the foundational expression of magnetostatics. Their theoretical style differed sharply from Ampère's: Biot stayed close to the experimental geometry, treating the magnetic pole as the basic interaction; Ampère insisted on a unified field theory built from currents alone. The two approaches converged a generation later in Maxwell's equations.

Biot's interests then pivoted to optics, and in 1815 he had already discovered that solutions of certain organic compounds — sugars, tartaric acid, camphor — rotate the plane of polarised light passing through them, in proportion to concentration and path length. This phenomenon, which he called optical activity, became the foundation of modern stereochemistry; Pasteur's discovery of molecular chirality in 1848 began as a follow-up to Biot's measurements on tartrate crystals. Biot remained scientifically active well into his eighties, publishing a textbook on celestial mechanics, translating the work of Egyptian astronomers from Arabic, and serving as a senator of the Second Empire. He died in Paris in 1862 at the age of eighty-seven, the last surviving member of the generation that had built quantitative French physics out of the wreckage of the Revolution.