# Poisson, Siméon Denis (1781–1840)

Siméon Poisson was a French mathematician and scientist whose main interest lay in the application of mathematics to physics, especially in electrostatics and magnetism. He developed a two-fluid theory of electricity and provided theoretical support for the experimental results of others, notably Charles de Coulomb. Poisson also made important contributions to mechanics, especially the theory of elasticity; to optics; to the calculus, especially definite integrals; to differential geometry; and to probability theory.

Poisson extended the work of Joseph Louis Lagrange on celestial mechanics and worked on integration and the Fourier series. In *Researches on the Probability of Opinions* (1837), he presented the **Poisson distribution**, which gives the probability of the occurrence of a specific event among a large number of events. **Poisson's ratio**, which relates lateral contraction to longitudinal extension in stressed materials, is also named after him. In all he wrote more
than 300 papers on mathematics, physics, and astronomy, and his *Traité
de mécanique* (1811) was long a standard work.