Poisson, Siméon-Denis (1781–1840)

French applied mathematician and physicist. In probability theory he formulated the Poisson distribution. Poisson's ratio in elasticity is the ratio of the lateral contraction of a body to its longitudinal extension. The ratio is constant for a given material.

Much of Poisson's work involved applying mathematical principles in theoretical terms to contemporary and prior experiments in physics, particularly with reference to electricity, magnetism, heat, and sound. Poisson was also responsible for a formulation of the ‘law of large numbers’, which he introduced in his work on probability theory, Recherches sur la probabilité des jugements/Researches on the Probability of Opinions (1837).

Poisson was born in Pithiviers, Loirel, and studied in Paris at the Ecole Polytechnique, where he became professor in 1806. In 1808 he was appointed astronomer at the Bureau des Longitudes, and the following year he was appointed professor of mechanics at the Faculty of Sciences. From 1820 he was an administrator at the highest level in France's educational system.

Poisson's works include Treatise on Mechanics (1833), Mathematical Theory of Heat (1835), and Researches on the Movement of Projectiles in Air (1835), the first account of the effects of the Earth's rotation on motion.