Ampère Reveals Magnetism’s Relationship to Electricity

André Ampère was the first scientist to describe the mathematical relationships between electricity and magnetism, or electrodynamics. His findings led to the modern understanding of light waves and radio waves and to the development of the telegraph.

Summary of Event

The magnetic compass was invented by the Chinese, who used lodestone, a naturally occurring magnetic material, in water compasses to guide ships as early as the eleventh century. Until the early nineteenth century, it was believed that only naturally occurring iron Iron;and magnetism[Magnetism] or lodestone was magnetic. In 1820, Danish physicist Hans Christian Ørsted performed a series of science demonstrations in his home for a group of his friends and students. First, Ørsted demonstrated that electric currents caused wires to heat up. He also planned to demonstrate magnetism and mounted a compass needle on a wooden stand. While performing his heating demonstration, Ørsted noticed that each time the electric current was turned on, the compass needle moved, suggesting that the electric current in the wire caused the deflection of the magnetic needle. This experiment provided the first demonstration that there was a relationship between electricity and magnetism. Ampère, André-Marie
Magnetism;and electricity[Electricity]
Electricity;and magnetism[Magnetism]
Mathematics;and electromagnetism[Electromagnetism]
[kw]Ampère Reveals Magnetism’s Relationship to Electricity (Nov. 6, 1820)
[kw]Reveals Magnetism’s Relationship to Electricity, Ampère (Nov. 6, 1820)
[kw]Magnetism’s Relationship to Electricity, Ampère Reveals (Nov. 6, 1820)
[kw]Relationship to Electricity, Ampère Reveals Magnetism’s (Nov. 6, 1820)
[kw]Electricity, Ampère Reveals Magnetism’s Relationship to (Nov. 6, 1820)
Ampère, André-Marie
Magnetism;and electricity[Electricity]
Electricity;and magnetism[Magnetism]
Mathematics;and electromagnetism[Electromagnetism]
[g]France;Nov. 6, 1820: Ampère Reveals Magnetism’s Relationship to Electricity[1140]
[c]Physics;Nov. 6, 1820: Ampère Reveals Magnetism’s Relationship to Electricity[1140]
[c]Science and technology;Nov. 6, 1820: Ampère Reveals Magnetism’s Relationship to Electricity[1140]
[c]Mathematics;Nov. 6, 1820: Ampère Reveals Magnetism’s Relationship to Electricity[1140]
[c]Radio and television;Nov. 6, 1820: Ampère Reveals Magnetism’s Relationship to Electricity[1140]
Ørsted, Hans Christian
Savary, Félix
Biot, Jean-Baptiste
Savart, Félix
Arago, François

François Arago Arago, François , a French physicist and astronomer, reported on Ørsted’s Ørsted, Hans Christian discovery at a meeting of the Academy of Sciences in Paris in September, 1820. Arago repeated Ørsted’s experiments at an academy meeting and began his own research on the relationship between electricity and magnetism. Just one week later, Arago showed that the passage of an electric current through a cylindrical spiral of copper wire caused it to attract iron filings as if it were a magnet. As soon as the current was turned off, the iron filings fell from the wire. Arago’s demonstration was the first use of an electromagnet, a magnet that functions because of the passage of current through a coiled wire.

Another French physicist, André Ampère, a professor of mathematics at the École Polytechnique in Paris, was fascinated by Arago’s report of Ørsted’s research. Although Ampère was primarily a mathematician, he also worked in a variety of other fields, including metaphysics, physics, and chemistry. He tried not only to repeat and extend Ørsted’s Ørsted, Hans Christian experiments but also to develop mathematical laws describing the relationship between electricity and magnetism. Ampère is not recognized as a methodical experimentalist but is known for having brilliant flashes of insight that he pursued to logical conclusions. Within a few weeks, Ampère demonstrated various electrical and magnetic effects to the academy. He recognized that if a current in a wire exerted a magnetic force on a compass needle, then two current-carrying wires should each produce a magnetic field, and the magnetic fields of these wires should interact. By the end of September, 1820, Ampère demonstrated these interactions, observing that two parallel, current-carrying wires are attracted to each other if both currents are in the same direction, and that they repel each other when the two currents flow in opposite directions.

Ampère’s discoveries allowed him to design and build an instrument called a galvanometer to measure the flow of electricity. A simple galvanometer is a compass with a conducting wire wrapped around it. When the wire carries an electrical current—as when a wire connects battery Batteries, electrical terminals—then the current that flows in the wire produces a magnetic field that deflects the compass needle. The stronger the current the larger the deflection of the needle; the position of the needle indicates the amount of current flowing in the wire. Ampère’s invention of the galvanometer led him to perform quantitative experiments on the relationship between the amounts of current flowing in pairs of wires and the strength of the magnetic forces between them. This work was critical in the formulation of the equations that relate electricity to magnetism.

Ampère was not the only person who reacted quickly to Arago’s report of Ørsted’s Ørsted, Hans Christian discovery. Jean-Baptiste Biot Biot, Jean-Baptiste and his assistant, mathematician Félix Savart Savart, Félix , conducted experiments on electromagnetism and reported to the Paris academy in October, 1820. This led to Biot-Savart’s law Biot-Savart’s law[Biot Savarts law] , which relates the intensity of the magnetic field set up by a current flowing through a wire to the distance from the wire. Another French experimenter who worked on magnetism at that time was Siméon-Denis Poisson, who treated magnetism as a phenomenon completely separate from electricity. Ampère continued his own work as well, describing his law for the addition of “electrodynamical forces” to the academy on November 6, 1820.

During the next few years Ampère was assisted by Félix Savary, Savary, Félix who performed many experiments and helped Ampère write up the results. Ampère’s most important publication on electricity and magnetism, Théorie des phénomènes électro-dynamiques (1826; theory of electrodynamic phenomena), describes four of his experiments and contains the mathematical derivation of the electrodynamic force law. Physicists now refer to one of Ampère’s mathematical relationships as Ampère’s law Ampère’s law[Amperes law] , an equation relating the electric current flowing through wires to the strength of the resulting magnetic fields at any distance from the wires.

Ampère also attempted to explain the natural magnetism of compass needles. He knew that when current flows through circular loops of wire, it creates magnets much like those of magnetic compass needles. Noting this, Ampère proposed that each atom of iron Iron;and electricity[Electricity] contains electric current, turning the atom into a small magnet. In iron magnets, these atomic magnets line up in the same direction, so their total magnetic forces are cumulative.


André Ampère’s discoveries, as well as François Arago’s Arago, François work, had immediate and practical applications. Once it was discovered that current-carrying wires generate magnetism, it was a simple matter to bend wires into coils that stack many loops of wire on top of one another and strengthen the overall magnetic effect. This finding led to the development of the electromagnet. In 1823, English electrical engineer William Sturgeon Sturgeon, William wrapped eighteen turns of copper wire around a bar, producing an electromagnet that could lift twenty times its own weight. In 1829, Joseph Henry Henry, Joseph used insulated wire on his electromagnet, allowing the wires to come closer together without shorting. By 1831, he demonstrated an electromagnet that could lift a ton of iron.

The electromagnet is also the basis for the operation of the telegraph, Telegraph;and electromagnetism[Electromagnetism] the first practical means for instant communication over long distances. Samuel F. B. Morse Morse, Samuel F. B. developed the idea of an electromagnetic telegraph in 1832. Although Morse constructed an experimental version in 1835, the first practical telegraph system was a line from Baltimore to Washington, D.C., that did not begin operation until 1844.

Ampère’s discovery also provided an explanation for the earth’s magnetic field, arguing that natural lodestone in the earth loses its magnetism at high temperatures, and temperature is known to increase with depth in the earth. A circulating electric current in the earth’s core is believed to generate the earth’s magnetic field. Finally, the discovery of the link between electricity and magnetism was fundamental to the later understanding of electromagnetic waves, including light waves and radio Radio waves. In 1864, James Clerk Maxwell Maxwell, James Clerk demonstrated that the connection between the electric and the magnetic forces involved a constant, the velocity of light in a vacuum. The idea that light was an electromagnetic phenomenon evolved from Maxwell’s work, which led to the discovery of radio waves Radio;waves , the development of the theory of relativity, and much of twentieth century physics. The fundamental unit of electric current was named the ampere in honor of Ampère’s contributions to electromagnetism.

Further Reading

  • Asimov, Isaac. Understanding Physics: Light, Magnetism, and Electricity. New York: Signet Books, 1966. This volume in Asimov’s history of physics includes a chapter on electromagnetism describing Ampère’s discoveries and their practical applications.
  • Darrigol, Oliver. Electrodynamics from Ampère to Einstein. New York: Oxford University Press, 2000. A 532-page exploration of the development of electrodynamics, beginning with Ampère’s experiments and the formulation of this new field during the early 1820’s. A well-documented and well-illustrated account of how Ampère’s work, and that of his successors, paved the way for Einstein’s theory of relativity.
  • Hofmann, James R., David Knight, and Sally Gregory Kohlstedt, eds. André-Marie Ampère: Enlightenment and Electrodynamics. New York: Cambridge University Press, 1996. A 420-page biography of Ampère, describing his significant contributions to mathematics, chemistry, and physics as well as his development of the new field of electrodynamics.

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