# Andre Marie Ampere

An Ampere is the standard unit of electric current. The current flowing in each of two parallel wires one meter apart is one ampere when the force between the wires is 2.1 x 10-7 newtons per meter. Before 1948 the ampere was defined in terms of the coulomb and was equal to the flow of one coulomb of charge per second. The coulomb has been redefined in terms of the ampere in such a way that this relationship still holds. The new (absolute) ampere is about 0.02% greater than the old (International) ampere.

Andre Marie Ampere was a French physicist and applied mathematician, who helped to establish the science of electrodynamics. He made experiments and formulated laws on the electromagnetic forces between conductors carrying electric currents.

Ampere was born in Lyon on January 20, 1775. Most of his education was private study under the guidance of his father, a wealthy silk merchant. At 14, Ampere had unusual competence in mathematics and a broad knowledge of science. In 1793, during the Reign of Terror, Ampere's father was executed.

After a happy marriage in 1799 and the birth of a son in 1800, he became a professor of physics in Bourg (1801-1803). A mathematical paper on game theory then gained him a post at the lycee in Lyon. On his wife's death he left in 1804 for the Ecole Polytechnique in Paris, and he taught in Paris for the rest of his life. He was elected to the Academy of Science in 1814.

At a meeting on September 11, 1820, the Academy was told about H.C. Oersted's discovery that a compass needle deflects when it is near a wire carrying an electric current. At subsequent meetings in September and October, Ampere showed that two nearby conductors carrying electric currents exert forces on each other, and he developed the mathematical laws for calculating such forces. He demonstrated that a long helical coil carrying current was magnetically similar to a bar magnet, and he showed that a piece of iron inserted in such a coil became strongly magnetized. He interpreted the magnetic forces between permanent magnets as forces between molecular electric currents in the iron. Initial criticism that part of his theory contradicted Newton's law of equality of action and reaction soon subsided, and Ampere's laws of electrodynamics gained universal acceptance and admiration. These laws became a basic element of J.C. Maxwell's electromagnetic theory in 1865.

Ampere, who was an ingenious experimenter, proposed a current-measuring galvanometer and a 26-wire electromagnetic telegraph, both based on Oersted's observation. He also suggested the rotating switch (commutator) first used on H. Pixii's electric generator in 1832. Ampere made other contributions in the fields of mechanics, optics, statistics, chemistry, and crystallography. He died in Marseille on June 10, 1836.

The ampere, the unit of electric current, is named after him. A law, now called Ampere's law, gives a quantitative expression for the force between two arbitrarily oriented current elements in a uniform medium.

Ampere's publications include Recueil d'ob-servations electrodynamiques (1822) and Theorie des phenomenes electrodynamiques (1826).

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