Ampère, André-Marie (1775–1836)

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AMPÈRE, ANDRÉ-MARIE (1775–1836)

André-Marie Ampère was born in Lyons, France, the son of a wealthy merchant. Ampère's education was determined by his father, Jean-Jacques, who followed Jean Jacques Rousseau's theories of education. Ampère was left to educate himself, as his inclinations dictated, among the books of his father's extensive library. At an early age Ampère discovered a talent for mathematics, working out the early books of Euclid by himself. On finding that some of the books he wished to consult in the library in Lyons were in Latin, he taught himself the language. Ampère's mother was a devout Catholic, who ensured he was thoroughly instructed in the faith.

Ampère's domestic life was beset with tragedy. In 1787 the French Revolution began; Jean-Jacques assumed the post of Juge de Paix, a role with considerable police powers. When Lyons fell to the troops of the Republic in 1793, Jean-Jacques was tried and guillotined. In 1799 Ampère married, supporting his wife by teaching mathematics in Lyons, where their son was born the next year. Weakened by childbirth, his wife died in 1803. Ampère moved to Paris and took a post in mathematics at the École Polytechnique. In 1806 he remarried, but this union was ill-advised. After the birth of a daughter, his wife and mother-in-law made life for Ampère so unbearable that he was forced to seek a divorce. Ampère persuaded his mother and sister to come to Paris and take charge of his household. Ampère's expectations of his children were never realized, and in domestic life he faced constant money problems. In 1808 Ampère was named Inspector General of the newly formed university system, a post he held until his death. In 1824 Ampère was elected to the Chair of Experimental Physics at the Collège de France.

As a deeply religious person whose personal life was beset by a series of calamities, Ampère searched in the field of science for certainty. He constructed a philosophy that enabled him to retain a belief in the existence of both God and an objective natural world. Ampère's philosophy contained two levels of knowledge of the external world. There are phenomena witnessed through the senses, and the objective causes of phenomena—noumena—that can only be apprehended through intellectual intuition. Although Ampère's philosophical system was the one continuing intellectual passion of his life, he also devoted himself to other fields of scientific research. From 1800 to around 1814, mathematics was Ampère's primary interest, with his spare time spent in chemical investigations. From 1820 to 1827 he carried out the scientific work for which he is best known, pioneering the science of electrodynamics.

In 1820 Hans Christian Oersted discovered electromagnetism. A report of Oersted's work was delivered before a sceptical meeting of the Académie des Sciences held on September 4, 1820. Oersted's work was contrary to established ideas, based on Coulomb's work of the 1780s, that there could not be any interaction between electricity and magnetism. Ampère however, immediately accepted Oersted's discovery, and set to work, reading his first paper on the subject to the Académie on September 18, 1820. Oersted's discovery suggested to Ampère, that two wires, each conducting current, might effect one another. Deducing the pattern of magnetic force around a current carrying wire to be circular, Ampère went on to visualize the resultant force if the wire were coiled into a helix. One week later Ampère announced to the Académie, his discovery of the mutual attraction and repulsion of two helices. In doing so Ampère presented a new theory of magnetism as electricity in motion.

Ampère's researches followed his own philosophy on the nature of science and scientific explanation. The phenomenon of electromagnetism had been discovered by Oersted, and the relationship between two current-carrying wires by Ampère; what remained was the discovery of the noumenal causes of the phenomenon. In his first memoir on electodyamics Ampère investigated the phenomenon and provided factual evidence to show that magnetism was electricity in motion. He concluded that two electric currents attract one another when moving parallel to one another in the same direction; they repel each other when they are parallel but in opposite directions. Ampère felt that electrical phenomena could be explained in terms of two fluids, a positive one flowing in one direction and a negative fluid going in the other.

In 1820 Ampère described the magnetism of a needle placed within a helical coil. With the assistance of Augustin Fresnel, Ampère unsuccessfully attempted to reverse the procedure by wrapping a coil around a permanent magnet. They did not investigate the effect of moving the magnet within the coil. If magnetism is only electricity in motion, then, Ampère argued, there must be currents of electricity flowing through ordinary bar magnets. It was Fresnel who pointed out the flaw in Ampère's noumenal explanation. Since iron was not a good conductor of electricity, there should be some heat generated, but magnets are not noticeably hot. In a letter to Ampère, Fresnel wrote, since nothing was known about the physics of molecules, why not assume that currents of electricity move around each molecule. Ampère immediately accepted the suggestion, assuming each molecule to be traversed by a closed electric current free to move around its center. The coercive force, which is low in soft iron, but considerable in steel, opposes this motion and holds them in any position in which they happen to be. Magnetism consists in giving these molecular currents a parallel direction; the more parallel the direction, the stronger the magnet. Ampère did not say why molecules should act this way, but it was sufficient that this electrodynamic model provided a noumenal foundation for electrodynamic phenomena.

In 1821 Michael Faraday sent Ampère details of his memoir on rotary effects, provoking Ampère to consider why linear conductors tended to follow circular paths. Ampère built a device where a conductor rotated around a permanent magnet, and in 1822 used electric currents to make a bar magnet spin. Ampère spent the years from 1821 to 1825 investigating the relationship between the phenomena and devising a mathematical model, publishing his results in 1827. Ampère described the laws of action of electric currents and presented a mathematical formula for the force between two currents. However, not everyone accepted the electrodynamic molecule theory for the electrodynamic molecule. Faraday felt there was no evidence for Ampère's assumptions and even in France the electrodynamic molecule was viewed with skepticism. It was accepted, however, by Wilhelm Weber and became the basis of his theory of electromagnetism.

After 1827 Ampère's scientific work declined sharply; with failing health and family concerns, he turned to completing his essay on the philosophy of science. In 1836 Ampère died alone in Marseilles during a tour of inspection.

Robert Sier