André-Marie Ampère and Electromagnetism
André-Marie Ampère was a prominent French physicist whose work laid foundational principles for electromagnetism, a branch of physics that studies the interaction between electric currents and magnetic fields. Ampère's interest in this field was sparked by the earlier discoveries of Hans Christian Ørsted, who observed that electric currents could influence magnetic compass needles. In 1820, Ampère conducted a series of experiments that demonstrated how two parallel wires carrying electric currents could attract or repel each other, depending on the direction of the currents. This work led him to formulate mathematical laws, notably Ampère's law, which describes the relationship between electric current and the strength of magnetic fields.
Ampère's innovations also included the invention of the galvanometer, an instrument used to measure electrical current through the magnetic effect it produces. His research significantly contributed to the development of practical applications such as electromagnets, which have since been used in various technologies, including telegraphs and electric motors. Beyond practical uses, Ampère's findings paved the way for further advancements in understanding electromagnetic waves, influencing later theorists like James Clerk Maxwell. The unit of electric current, the ampere, is named in his honor, reflecting his lasting impact on the field of electromagnetism.
André-Marie Ampère and Electromagnetism
Date November 6, 1820
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.
Locale Paris, France
Key Figures
André-Marie Ampère (1775-1836), French mathematician and physicistHans Christian Ørsted (1777-1851), Danish physicistJean-Baptiste Biot (1774-1862), French mathematician and physicistFélix Savart (1791-1841), French mathematician and physicistFrançois Arago (1786-1853), French physicist
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 or lodestone was magnetic. In 1820, Danish physicistHans 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.
François Arago, a French physicist and astronomer, reported on Ørsted’s 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 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 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 discovery. Jean-Baptiste Biot and his assistant, mathematician Félix Savart, conducted experiments on electromagnetism and reported to the Paris academy in October, 1820. This led to Biot-Savart’s 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, 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, 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 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.
Significance
André Ampère’s discoveries, as well as François Arago’s 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 wrapped eighteen turns of copper wire around a bar, producing an electromagnet that could lift twenty times its own weight. In 1829, Joseph Henry 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, the first practical means for instant communication over long distances. Samuel F. B. Morse 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 waves. In 1864, James Clerk Maxwell 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, 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.
Bibliography
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.