Siméon Denis Poisson (21.06.1781-25.06.1840) — French mechanician, mathematician and physicist, a member of the Paris Academy of Sciences (1812). Born in Pithiviers. Graduated from the École Polytechnique (1800), where he taught (a professor from 1806), and from 1809 was a professor at the University of Paris.
His physical research concerned electricity and magnetism, capillarity, the theory of elasticity, hydromechanics, the theory of oscillations and the theory of light. In 1811 he applied the mathematical theory of the potential to electrostatics, formulating here an important theorem according to which the field strength at a point near the surface of a conductor is proportional to the charge density on the conductor; and he solved the problem of the distribution of electricity on the surface of various conductors and systems of conductors. In 1824 he extended potential theory to magnetic phenomena and developed a mathematical theory of magnetism.
He was the first to write the equations of analytical mechanics in terms of the components of momentum, and in hydrodynamics he generalised the Navier–Stokes equation to the motion of a compressible viscous fluid with heat transfer taken into account. He developed a theory of the propagation of sound in air. He was the first to investigate the adiabatic process theoretically, and gave (1823) a derivation of the equation of the adiabat (Poisson's equation).
He solved a number of problems in the theory of elasticity and introduced (1810) the so-called “Poisson's ratio” as a characteristic of the material of an elastic body — the ratio of longitudinal extension to transverse contraction — and generalised the equations of elasticity theory to anisotropic bodies. Together with L. Navier and A. Cauchy he is a founder of the theory of elasticity.
A member of the St Petersburg Academy of Sciences (1826).