Remembered for his theoretical contributions in magnetism and electricity, Simeon-Denis Poisson was also credited with furthering the work in celestial mechanics of Pierre-Simon Laplace and Joseph-Louis Lagrange. Poisson made significant inroads in the field of probability as well, developing the Poisson distribution to describe the likelihood of a particular event.
Poisson was born on June 21, 1781 in Pithiviers, Loiret, France, the son of a low-ranking civil servant. He was a sickly child whose mother often put him in the care of a nurse. When he was old enough, Poisson was made an apprentice to an uncle who was a surgeon, but the young man showed neither talent for nor interest in the profession. When he was 18, Poisson enrolled at the Ecole Central in Fontainebleau, where his aptitude for mathematics and learning in general finally came to light after an early education of limited usefulness. In fact, he did so well that in 1798 he was admitted to the famous Ecole Polytechnique in Paris after graduating first in his class.
With such teachers as Laplace and Lagrange, who were immediately impressed by Poisson's mathematical prowess, Poisson progressed rapidly. Apart from his relative inability to draw acceptable diagrams, which precluded him from a career in descriptive geometry, Poisson was so gifted in mathematics that the school made him a teacher as soon as he graduated in 1800. He became an assistant professor in 1802 and in 1806 replaced the school's illustrious Jean Baptiste Joseph Fourier as full professor.
As word of his mathematical skill traveled, Poisson received many offers of employment. In 1808 he left the Ecole Polytechnique to accept an appointment as an astronomer at the Bureau des Longitudes and then as mechanics professor at the Faculty of Sciences in 1809. Two years later he published his Treatise on Mechanics, which became the standard reference for many years.
One of Poisson's most important works was a memoir in 1812 in which he expounded on the two-fluid theory of electricity. According to his modified version, similar fluids repel and dissimilar fluids attract based on what he called the inverse square law. The electrical charge of a body (i.e., positive or negative), then, depended on how the normally uniform distribution of both fluids became disrupted, causing a charge. Poisson adapted Lagrange's work on the subject, showing mathematically that this function would remain constant over the surface of an insulated conductor and gave a proof of the formula for the force at a charged conductor's surface. Some scientific historians consider that his work in this area launched a new branch of mathematical physics.
In 1815 Poisson suggested some changes to Fourier's theory of heat that served only to embitter the men's relationship, with Fourier accusing Poisson of wasting his talent on merely modifying the work of others. However, Poisson also built upon Lagrange's and Laplace's celestial mechanics--particularly concerning the stability of planets' orbits and calculations of the gravitational attraction exerted by ellipsoidal and spherical bodies--with good results. His expression for the gravitational force in terms of mass distribution inside a planet is still in use to calculate details of the Earth's shape based on paths of satellites in orbit.
Poisson accepted a nomination in 1820 to the Royal Council of the University, giving him the leverage and prestige he needed to defend science in France as a worthwhile and necessary discipline. (A new conservative government under the restored Louis XVIII was working to eliminate the scientific programs and policies implemented during the country's Revolutionary and Napoleonic periods.) After Lagrange's death in 1827, Poisson bore even more of this burden. Thus, the fact that he published almost all of his books in the last decade of his life is even more impressive.
In 1824, Poisson published a paper in which he discussed a theory of magnetism. This was based on the two-fluid model, but yielded a general expression for the magnetic potential as the sum of two integrals. In addition, he contributed Poisson's ratio to the theory of elasticity, which concerns the ratio of longitudinal extension to lateral contraction. He also wrote a memoir in 1833 on the movement of the Moon. In 1835, he published A Mathematical Theory of Heat.
The Poisson distribution rule (also known as the Poisson law of large numbers), which he introduced in the 1837 Research on the Probability of Opinions, represented one of his most significant efforts. The principle involves events that normally would be highly unlikely but that happen nevertheless due to the many chances for them to occur, such as plane crashes, for example. In modern times, the Poisson rule is helpful in the analysis of traffic, radioactivity, and random events in space or time. In the realm of pure mathematics, Poisson's most important contributions were papers on definite integers and Fourier series, which helped to launch the research of Bernhard Rieman and Peter Gustav Lejeune Dirichlet.
Poisson died on April 25, 1840 in Paris. He had married in 1817 and had been made a baron in 1837.
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