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André Marie Ampère: Discovery of the Link Between Electricity and Magnetism

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André Marie Ampère in 1825.

André Marie Ampère in 1825.

Who Was André Marie Ampère?

André Marie Ampère, a French physicist and mathematician, discovered the relationship between electricity and magnetism. He built some of the earliest electromagnets and developed a theory to explain magnetic effects. His work foreshadowed the discovery of the electron and its role in the modern atomic theory of matter. Ampère’s important contribution to the understanding of electricity and magnetism was recognized by naming the unit of electric current after him—the ampere. In recognition of his contributions to science, his name is one of the 72 names of French scientists, engineers, and mathematicians engraved on the Eiffel Tower in Paris.

Early Life

André Marie Ampère was born on January 22, 1775, in the village of Polémieux, near Lyons in France. As a young boy he was very bright, and his wealthy father paid to have him taught by private tutors. The young man also read extensively from his father’s library and taught himself much about science and mathematics. His interests turned to science and mathematics at an early age. As a youth he read the mathematical works of Leonhard Euler, Joseph Louis Lagrange, and Daniel Bernoulli, in their original Latin. By the time he was eighteen, he had read Pierre-Simon Laplace’s five-volume treatise on celestial mechanics titled Traité de mécanique celeste, and the two volume French treatise on analytical mechanics written by Joseph-Louis Lagrange, Mécanique analytique.

Ampère’s studies were interrupted in 1793 when Lyons revolted against the revolutionaries of the French Revolution and the city was taken by the republican army. Ampère’s father, a successful merchant and one of the city’s officials, found himself on the wrong side of the revolutionary conflict and was put to the guillotine. This tragedy sent Ampère into depression and weighed heavily on him for years to come.

To fund his continued studies in science and mathematics, in about 1796 he gave private instruction in mathematics, chemistry, and other languages at Lyons. In 1799 he began a mathematics teaching post that provided the income and stability that he needed to marry and father a son. In 1802, he gained an appointment at the École Centrale in Bourg-en-Bresse as professor of physics and chemistry. To obtain the position, he had to leave behind his ailing wife. She died three years later, a loss that haunted him the rest of his life.

Academic Career

After a publication on mathematical theory of games of chance and having successfully completed some experimental work on electrical machines, in 1804 he moved to an appointment at the Lyceum at Lyons. A year later, he was granted a lectureship in mathematics at the Ecôle Polytechnique in Paris. At the Polytechnique, he flourished intellectually and delved deeper into his studies in mathematics and physics. By 1809, he had advanced to the rank of full professor, a position he would hold until his death. The French ruler Napoleon Bonaparte recognized Ampère’s talent and appointed him inspector general of the newly formed university system.

Development of Electromagnetics

In September 1820, a colleague of Ampère demonstrated the recent discovery by the little-known Danish Physicist Hans Christian Oersted at a meeting of the French Academy of Science. Oersted had shown the deflection of a compass needle when placed in the vicinity of an electric current. Oersted’s discovery was the first to suggest a link between electricity and magnetism, two of the most mysterious natural phenomena at the time.

Inspired by the demonstration, Ampère plunged into a flurry of experimentation and within a week was able to submit his developments to the Academy. Over the next four months he gave a series of presentations to the academy, developing a mathematical theory to explain the interaction between electricity and magnetism. He coined the term “electrodynamics,” which is the study of the movement of electrical charges. Today the more commonly used term is electromagnetism. A similar term is “electrostatics,” which is the study of fixed electrical charges and their interactions.

Oersted’s apparatus to show the electric current causes a deflection in the compass needle.

Oersted’s apparatus to show the electric current causes a deflection in the compass needle.

The “Right-Hand Rule” in Electromagnetics

In his research, Ampère had shown that a wire carrying a current is suspended horizontally along a north-south line when a magnet is brought under it to deflect to the east. He showed that the deflection of the needle could be expressed by what we now call the “right-hand rule.” Here, the right hand is imagined as grasping a current carrying wire, with the thumb pointing in the direction of the current. The curled fingers then indicate the direction the north pole of the magnet will be deflected.

Right Hand Rule. When the thumb is placed in the direction of the electrical current (I), then the curled fingers indicate the direction of the resulting magnetic field (B).

Right Hand Rule. When the thumb is placed in the direction of the electrical current (I), then the curled fingers indicate the direction of the resulting magnetic field (B).

The Relationship Between Electric and Magnetic Fields

Ampère knew that electrical currents affect a magnet, and a magnet affects another magnet. He then wondered, should not one current affect another current? He next proceeded to test his hypothesis experimentally be arranging two wires, each part of a separate electrical circuit, to dangle freely parallel to each other. Using switches, he was able to control the direction of the electric current in the two wires. Ampère showed that when the direction of the current between the two wires was in the same direction, the two wires moved toward each other, and when the direction of the current in the two wires moved in opposite directions, the two parallel wires were repulsed and moved away from each other.

He went on to prove that the force of attraction or repulsion between the two wires was directly proportional to the amount of current flowing in the wires, and the force was inversely related to the square of the distance between the two wires. This became known as Ampère’s force law.

Diagram illustrating the attractive force between two wires that carry the electrical current in the same direction and the repulsive force when wires carry the current in opposite directions.

Diagram illustrating the attractive force between two wires that carry the electrical current in the same direction and the repulsive force when wires carry the current in opposite directions.

Publications and Legacy

Ampère published his work on electricity and magnetism in a series of papers in Annales de chimie et de physique (French for Annals of Chemistry and of Physics) during the 1820s. In 1827, Ampère published his magnum opus, the Memoir on the Mathematical theory of Electrodynamical Phenomena, Uniquely Deduced from Experimentation, in which he defined the science of electromagnetism. He would continue his studies into mathematics and the philosophical aspect of science as his health began to fail. He died in Marseilles, France, on June 10, 1836.

It was Ampère’s discovery of the inverse-square law relationship in electromagnetics that prompted the eminent physicist James Clerk Maxwell to refer to Ampère as the “Newton of electricity.” Ampère’s work in electricity and magnetism became key to nineteenth-century theoretical and practical developments in electromagnetics.

References

  • A Dictionary of Scientists. Oxford: Oxford University Press.
  • Asimov, Isaac. Asimov’s Biographical Encyclopedia of Science and Technology, 2nd Revised Edition. Doubleday & Company, Inc., 1982.
  • Hart, Ivor B. “André Marie Ampère.” Collier Encyclopedia. 1966.
  • Williams, L. Pearce. “Ampère, André-Marie” in Dictionary of Scientific Biography, Vol. 1, pp. 139-147, New York: Charles Scribner’s Sons, 1981.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

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