3.1 The Discovery of Electromagnetism

How Electric Currents Give Rise to Magnetic Effects

Mar 06, 2024

Lightning was known to sometimes impart a permanent magnetization on steel objects it struck, yet a direct demonstration of the relationship between electricity and magnetism remained elusive. In 1819, Hans Christian Örsted (1777–1851) finally proved the connection between electricity and magnetism in a simple, repeatable experiment. Figure 3.2 shows the Danish physicist and his apparatus. Running an electric current through a wire adjacent to a magnetic compass needle, Örsted demonstrated the current perturbed the compass needle [[iii]]. Clearly, there was some physical connection between electricity and magnetism. Örsted communicated his discovery, saying “the electric conflict acts in a revolving manner” [[iv]]. Before long, investigators all over Europe were repeating Örsted’s experiment and trying to figure out what it meant. Electricity and magnetism were no longer two separate fields, and electromagnetism was born.

Figure 3.2: Hans Christian Örsted (1777-1851) [**[i]**], left, and his apparatus showing electric currents yield magnetic effects [**[ii]**], right.

The French savant, physicist, and mathematician André Marie Ampère (1775–1836) was a precocious child. He disdained the study of Latin and threw himself into mathematics and algebra. Told at age 12 that the differential calculus books he wished to study were written in Latin, he nevertheless resolved to master the language, and he did [[v]].

Ampère took a variety of teaching jobs to support his son and his ailing wife. After her death in 1803, he obtained a position at the École Polytechnique in Paris.

In 1820, news of Örsted’s discovery reached Paris. Ampère was struck by a deep insight – that magnetism was merely moving electricity. Since a current moved a magnetic needle, two parallel currents ought to feel forces of attraction or repulsion. Ampère was correct. He launched into a series of experiments to quantify his intuition, and he demonstrated that parallel currents attract and anti-parallel currents repel with a force proportional to the inverse distance squared. Figure 3.3 shows Ampère and his experimental apparatus.

Figure 3.3: André-Marie Ampère (1775–1836) [**[vi]**], left, and the table he used to measure the forces of magnetic attraction between electric currents [**[vii]**], right.

Some critics disputed Ampère’s interpretation of his findings, arguing that the interactions he found between currents were purely electrical, not magnetic. Others insisted that his results were obvious in light of Örsted’s work. The power and precision of Ampère’s results soon overcame his critics. In particular, the similarity between Ampère’s result for magnetic attraction, Coulomb’s electrostatic force law, and Newton’s Law of Universal Gravitation was both obvious and persuasive.

Writing a half century later, Maxwell declared:

The experimental investigation by which Ampère established the laws of the mechanical action between electric currents is one of the most brilliant achievements in science.
The whole theory and experiment seems as if it had leaped, full grown and full armed, from the brain of the ‘Newton of electricity.’ It is perfect in form, and unassailable in accuracy, and it is summed up in a formula from which all the phenomena may be deduced, and which must always remain the cardinal formula of electro-dynamics [[viii]].

Coulomb, Ampère, and their contemporaries viewed electricity as the study of point charge interactions. They presumed actions arose at a distance, without any go-between. As Maxwell noted:

...Cavendish, Coulomb, and Poisson, the founders of the exact sciences of electricity and magnetism... turned their undivided attention to the determination of the law of force, according to which electrified and magnetized bodies attract or repel each other. In this way, the true laws of these actions were discovered, and this was done by men who never doubted that the action took place at a distance, without the intervention of any medium, and who would have regarded the discovery of such a medium as complicating rather than as explaining the undoubted phenomena of attraction [[ix]].

Working these premises, physicists developed many of the mathematical tools used to this day to describe electromagnetic systems. Having discovered additional inverse-square-law forces of attraction, they applied what they thought they knew of gravity to the new fields of electricity and magnetism. When they did so, the philosophic premises they held regarding gravitation were carried through into the new theories. They interpreted electricity and magnetism as point charges interacting at a distance via mathematical relationships with no causal mechanism.

It would take an unconventional thinker to defy the conventional wisdom and conclude instead that “…action at a distance can only result through an action of the contiguous conducting particles” [[x]]. That unconventional thinker was Michael Faraday (1791–1867).

Next time, 3.2 Michael Faraday.

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References

[i] Image from/fra J. P. Trap: berømte danske mænd og kvinder, 1868 See: https://commons.wikimedia.org/wiki/File:HC_%C3%98rsted.jpg.

[ii] Downloaded 13 August 2013 from Agustin Privat-Deschanel 1876 Elementary Treatise on Natural Philosophy, Part 3: Electricity and Magnetism, D. Appleton and Co., New York, translated by J. D. Everett, p. 656, fig. 456 on Google Books See https://commons.wikimedia.org/wiki/File:Oersted_experiment.png

[iii] O’Reilly, Michael Francis (writing as Brother Potamian), and James Joseph Walsh, Makers of Electricity, New York: Fordham University Press, 1909, pp. 205-231.

[iv] James Clerk Maxwell, "Action at a Distance," Proceedings of the Royal Instituion of Great Britain, Vol. 7, 1873-5, London, p. 49. As quoted in The Scientific Papers of James Clerk Maxwell (W.D. Niven, ed.), (New York: Dover, 1953?) pp. 317. Originally published, 1890.

[v] O’Reilly, Michael Francis (writing as Brother Potamian), and James Joseph Walsh, Makers of Electricity, New York: Fordham University Press, 1909, p. 232-257.

[vi] André Marie Ampère. Reproduction of lithograph by J. Boilly. Credit: Wellcome Collection. CC BY

[vii] Table with devices that demonstrate electrodynamic laws devised by Ampère; in the Collège de France, Paris. Photoprint. Credit: Wellcome Collection. CC BY

[viii] Maxwell, James Clerk, A Treatise om Electricity and Magnetism, vol. 2, p. 162.

[ix] Maxwell, James Clerk, “Action at a Distance,” Proceedings of the Royal Instituion of Great Britain, Vol. 7, 1873-5, London, p. 49. As quoted in The Scientific Papers of James Clerk Maxwell (W.D. Niven, ed.), (New York: Dover, 1953?) pp. 317. Originally published, 1890.

[x] Faraday, Michael, Experimental Researches in Electricity, vol. 1, London: Bernard Quaritch, 15 Picadilly, 1839, §1680, p. 535.