Ampère, André-Marie (1775–1836)

French physicist and mathematician who made many discoveries in electromagnetism and electrodynamics. He followed up the work of Hans Oersted on the interaction between magnets and electric currents, developing a rule for determining the direction of the magnetic field associated with an electric current. The unit of electric current, the ampere, is named after him.

Ampère's law is an equation that relates the magnetic force produced by two parallel current-carrying conductors to the product of their currents and the distance between the conductors. Today Ampère's law is usually stated in the form of calculus: the line integral of the magnetic field around an arbitrarily chosen path is proportional to the net electric current enclosed by the path.

Ampère was born near Lyon and taught mathematics there until moving to Paris 1805. He held academic posts in mathematics, philosophy, astronomy, and experimental physics.

Ampère's first publication was an early contribution to probability theory, Considérations sur la théorie mathématique de jeu/Considerations on the Mathematical Theory of Games 1802, in which he discussed the inevitability of a player losing a gambling game of chance against an opponent with vastly greater financial resources.

In a series of papers beginning 1820 Ampère expounded the theory and basic laws of electromagnetism (which he called electrodynamics to differentiate it from the study of stationary electric forces, which he called electrostatics). He showed that two parallel wires carrying current in the same direction attract each other, whereas when the currents are in opposite directions, mutual repulsion results. He also predicted and demonstrated that a helical coil of wire (which he called a solenoid) behaves like a bar magnet while it is carrying an electric current.

Trying to explain electromagnetism, Ampère proposed that magnetism is merely electricity in motion. His suggestion that molecules are surrounded by a perpetual electric current may be regarded as a precursor of the electron-shell model.

He published Mémoire sur la théorie mathématique des phénomènes électrodynamiques uniquement déduite de l'expérience/Notes on the Mathematical Theory of Electrodynamic Phenomena Deduced Solely from Experiment 1827.