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Summary Article: Ampère, André-Marie (1775-1836)
From The Hutchinson Dictionary of Scientific Biography

Place: France

Subject: biography, physics

French physicist, mathematician, chemist, and philosopher who founded the science of electromagnetics (which he named electrodynamics) and gave his name to the unit of electric current.

Ampère was born in Polémieux, near Lyon, on 22 January 1775. The son of a wealthy merchant, he was tutored privately and was, to a great extent, self-taught. His genius was evident from an early age, particularly in mathematics, which he taught himself and had mastered to an extremely high level by the age of about 12. The later part of his youth, however, was severely disrupted by the French Revolution. In 1793 Lyons was captured by the republican army and his father - who was both wealthy and a city official - was guillotined. Ampère taught mathematics at a school in Lyon from 1796 to 1801, during which period he married (in 1799); in the following year his wife gave birth to a son, Jean-Jacques-Antoine, who later became an eminent historian and philologist. In 1802 Ampère was appointed professor of physics and chemistry at the Ecole Centrale in Bourg, then, later in the same year, professor of mathematics at the Lycée in Lyon. Two years later his wife died, a blow from which Ampère never really recovered - indeed, the epitaph he chose for his gravestone was Tandem felix (‘Happy at last’). In 1805 he was appointed an assistant lecturer in mathematical analysis at the Ecole Polytechnique in Paris where, four years later, he was promoted to professor of mathematics. Meanwhile his talent had been recognized by Napoleon, who in 1808 appointed him inspector general of the newly formed university system, a post he retained until his death. In addition to his professorship and inspector-generalship, Ampère taught philosophy at the University of Paris in 1819, became assistant professor of astronomy in 1820 and was appointed to the chair in experimental physics at the Collège de France in 1824 - an indication of the breadth of his talents. He died of pneumonia on 10 June 1836 while on an inspection tour of Marseille.

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. It was on the strength of this paper that he was appointed to the professorship at Lyon and later to a post at the Ecole Polytechnique.

In the period between his arrival in Paris in 1805 and his famous work on electromagnetism in the 1820s, Ampère studied a wide range of subjects, including psychology, philosophy, physics, and chemistry. His work in chemistry was both original and topical but in almost every case public recognition went to another scientist; for example, his studies on the elemental nature of chlorine and iodine were credited to the English chemist Humphry Davy. Ampère also suggested a method of classifying elements based on a comprehensive assessment of their chemical properties, anticipating to some extent the development of the periodic table later in that century. And in 1814 he independently arrived at what is now known as Avogadro's hypothesis of the molecular constitution of gases. He also analysed Boyle's law in terms of the isothermal volume and pressure of gases.

Despite these considerable and varied achievements, Ampère's fame today rests almost entirely on his even greater work on electromagnetism, a discipline that he, more than any other single scientist, was responsible for establishing. His work in this field was stimulated by the finding of the Danish physicist Hans Christian Oersted that an electric current can deflect a compass needle - that is, that a wire carrying a current has a magnetic field associated with it. On 11 September 1820 Ampère witnessed a demonstration of this phenomenon given by Dominique Arago at the Academy of Sciences and, like many other scientists, was prompted to hectic activity. Within a week of the demonstration he had presented the first of a series of papers in which he 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.

In addition, Ampère reasoned that the deflection of a compass needle caused by an electric current could be used to construct a device to measure the strength of the current, an idea that eventually led to the development of the galvanometer. He also realized the difference between the rate of passage of an electric current and the driving force behind it; this has been commemorated in naming the unit of electric current the ampere (a usage introduced by Lord Kelvin in 1883). Furthermore, he tried to develop a theory to explain electromagnetism, proposing that magnetism is merely electricity in motion. Prompted by Augustus Fresnel (one of the originators of the wave theory of light), Ampère suggested that molecules are surrounded by a perpetual electric current - a concept that may be regarded as a precursor of the electron-shell model.

The culmination of Ampère's studies came in 1827, when he published his famous 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, in which he enunciated precise mathematical formulations of electromagnetism, notably Ampère's law - 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 produced little worthy of note after the publication of his Mémoire but his work had a great impact and stimulated much further research into electromagnetism.

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