André-Marie Ampère

French physicist

• Born: January 22, 1775; Lyon, France
• Died: June 10, 1836; Marseilles, France

André-Marie Ampère was a major French scientist of the early nineteenth century who, through extensive experimentation with electric currents, discovered the relationship between electricity and magnetism and established an early form of electrodynamics.

Primary field: Physics

Specialties: Electromagnetism; physical chemistry

Early Life

André-Marie Ampère (AM-peer) was born on January 22, 1775, into a wealthy French merchant family that was successful in the silk trade. The family moved between the rural village of Polémieux and Lyon, a busy center of trade. They belonged to the Roman Catholic Church, and Ampère himself remained a devoted Catholic throughout his life. As a boy, Ampère received private tutoring at home and was influenced by the writings of Antoine Léonard Thomas, René Descartes, Jean-Jacques Rousseau, and Georges-Louis Leclerc. He excelled at mathematics and showed an early interest in natural history and botany. During his childhood, Ampère became so involved in his study of mathematics that he began to formulate his own mathematical theories; infinitesimal magnitudes and differential calculus were among his early interests. The writings of French mathematician Jean le Rond d’Alembert, in particular, inspired him in his own investigations, as did those of mathematicians Daniel Bernoulli, Leonhard Euler, and Joseph-Louis de Lagrange.

By 1788, at the age of thirteen, Ampère submitted a mathematical paper on circle squaring to the Académie de Lyon. His father aided him in his quest to become more knowledgeable about mathematics and science, bringing him to the Collège de la Trinité.

The French Revolution erupted in 1789. The Republican army laid siege to Lyon in 1792 and 1793, a period that, due to the executions and widespread violence, became known as the Reign of Terror. The Ampère family was unable to escape the violence; Ampère’s father, Jean-Jacques Ampère, was executed on the guillotine because of his political position and bourgeois status. Ampère was physically and emotionally incapacitated by the news of his father’s death, withdrawing from his old pursuits. By 1795, however, Ampère had become interested in such scientific subjects as astronomy and mechanics. He conducted a number of experiments with telescopes and applied his mathematical knowledge to the gravitational relationships between the moon, the sun, and the Earth.

In 1799, Ampère married Catherine Carron. The couple had a son, Jean-Jacques, before Catherine’s death in 1803.

Life’s Work

Ampère’s first work on mathematics was an analysis of probability titled Considerations on the Mathematical Theory of Games, published in 1802, in which he examined the probability that a gambler who wagers the same fraction of his original purse in every game will be eliminated from play after any specific number of games. In his studies of chemistry, he arrived independently at Avogadro’s number (though a few years following the original discovery) and pursued an interest in the composition of chemical elements such as iodine.

In order to supplement his family’s income, Ampère worked in a number of academic positions. His first was in 1802 at the École Centrale in Bourg-en-Bresse where he taught physics and chemistry. After his wife died in 1803, Ampère moved to Paris, where he taught at the École Polytechnique. He was made professor of mathematics in 1809 and would remain at the school until 1828. Ampère also taught courses in philosophy and astronomy at the University of Paris.

Ampère became interested in what is known today as “Laplacian physics,” named after Pierre-Simon Laplace. Laplacian physics refers to the effort among scientists of the late eighteenth century to unify fields of science such as optics, heat, magnetism, and electricity. In response to the question of how material particles are affected by different forces, Laplacian physics emphasized the interactions between particles at short ranges, and viewed electric and magnetic forces as different entities. Yet Ampère did not fully accept these conclusions. In his early investigations of how electric and magnetic effects could be materially transmitted, Ampère arrived at two limiting principles. The first was that the observable properties in fluids (whether liquid or gas) can be attributed to electric and magnetic fluids; the second was that fluids and noncontiguous bodies do not necessarily react to each other. From these principles, Ampère concluded that one should focus on the activity between molecules of matter and their immediate surroundings.

In 1820, Ampère made a great breakthrough following the publication of a groundbreaking essay on electric currents by Danish physicist Hans Christian Ørsted. The essay, “An Experiment Concerning the Effect of the Interaction of Electricity with Magnets,” reported that an electric current could change the normal orientation of a suspended bar magnet. From Ørsted’s examples, Ampère concluded and would soon prove, that there was no such thing as “magnetic fluid”; electric charges were in fact responsible for magnetic effects.

Excited by the possibilities presented by Ørsted’s research, Ampère tested his hypothesis in a series of experiments. One of his most famous experiments involved using a pair of wires that carried electric currents. He found that magnetic attraction occurred between the wires when the currents were running in the same direction, and the wires with currents running in opposite directions repelled each other. For the first time, Ampère identified the role of electric currents in magnetic attraction and repulsion. Magnetic forces, he explained, originate in electric charges. He created four demonstrations of electromagnetic forces using an early design of a device called a galvanoscope, wire coils called solenoids, and bar magnets. Using these demonstrations, he tested his hypothesis of a law of electrodynamics.

In October 1820, Ampère began experimenting with coiled wire in the shape of a helix, or spiral. He was attempting to test magnetic forces and replicate the behavior of bar magnets. He was surprised to find that his helices were not affected by terrestrial magnetism. Working with the coiled wire allowed Ampère to see potential problems with his magnetism hypothesis and led him to an interest in equilibrium experiments. Through trial and error, Ampère was able to construct an apparatus that allowed him to test his force law theory. He created a circuit using vertical linear wire suspended from two conductors. One of the conductors was able to rotate around a vertical axis. The linear wire was subject to repulsive forces exerted by the conductors. Another wire twisted into a helix enabled the conductors to successfully exert these forces on the wire.

The results of these experiments led to Notes on the Mathematical Theory of Electrodynamic Phenomena Deduced Solely from Experiment (1827), Ampère’s greatest written contribution to the sciences. In this work, he uses mathematical equations to describe how electromagnetism functions. Ampère’s law is expressed by an equation that demonstrates the relationship between magnetic forces and the product of the currents that are produced by two parallel conductors; it also considers the relationship between magnetic forces and the distance between conductors. Ampère demonstrated that current-carrying linear conductors are like magnetic shells, and the strength of the magnetic field created depends upon the strength of the current.

Ampère’s work was not initially well received. Some scientists thought his theories contradicted those of English physicist Isaac Newton; others could not follow his analysis. His theories were clarified, however, by British scientist Charles Babbage, after which they were generally accepted.

After completing his 1827 treatise on electrodynamics, Ampère drifted away from his research. He died on June 10, 1836, from pneumonia.

Impact

André-Marie Ampère is known as the founder of electrodynamics (now known as electromagnetism), a branch of physical science that focuses on the relationship between electric currents, magnets, and the effects of their interactions. His greatest scientific breakthrough came in 1820, when he demonstrated the role of the forces of attraction and repulsion between electrical currents. He expressed this breakthrough in Ampère’s law, which states that the magnetic force between two wires is directly proportional to the length of the wires and the distance traveled by their respective electric currents. He published his most significant conclusions in Notes on the Mathematical Theory of Electrodynamic Phenomena Deduced Solely from Experiment. Electric currents are now measured in amperes, in honor of his discovery. Ampère’s work in electrodynamics led to the development of the galvanometer—a wire coil in a magnetic field that is used to measure the strength and direction of electric currents—and contributed to the invention of the electric telegraph. His suggestion that an electric current surrounds all molecules anticipated the electron shell model, which describes the electron groups orbiting an atom’s nucleus.

Bibliography

Darrigol, Olivier. Electrodynamics from Ampère to Einstein. New York: Oxford UP, 2003. Print. Follows the scientific investigations that accompanied Ampère’s early definition of electrodynamics and examines the developments of technology, method, and historical context surrounding scientists such as Michael Faraday, Heinrich Hertz, and Albert Einstein.

Flynn, George J. “Ampère Reveals Magnetism’s Relationship to Electricity.” Great Events from History: The Nineteenth Century. Ed. John Powell. Pasadena, CA: Salem, 2007. Print. Describes and provides further details on the historical background and circumstances of Ampère’s experiments in 1820.

Hofmann, James R. André-Marie Ampère: Enlightenment and Electrodynamics. New York: Cambridge UP, 1995. Print. Chronicles Ampère’s personal life and pursuit of study, from the French Revolution into the nineteenth century.

Keithley, Joseph F. The Story of Electrical and Magnetic Measurements: From 500 BC to the 1940s. New York: IEEE, 1999. Print. Details the scientific history surrounding research in electricity. Follows the accomplishments and discoveries of such figures as Ampère, Lord Kelvin, Joseph John Thomson.