Dominique Francois Jean Arago

French Physicist: worked on Optics and Magnetism


Arago became a professor of analytical geometry at the école Polytechnique in Paris at the age of 23. He served as secretary of the Académie des Sciences and the Bureau de Longitudes. Working with Biot, Arago completed the measurements of an arc of meridian in order to establish a standard of length, the metric system. For this mission, they traveled to Spain in 1806, on their return journey, they were shipwrecked and almost imprisoned in Algiers.


During his work with Fresnel, he discovered that two beams of light polarised in perpendicular directions do not interfere with each other, this implied that light consisted of transverse waves. This agreed more with Young’s Theory than with Fresnels research, and therefore Arago stopped working with Fresnel. His theory predicted that the velocity of light should decrease as it passes into a denser medium. The experiment to show this was carried out by Fizeau and Léon Foucault as Arago eye sight was poor.


In 1819, Hans Christian Orsted, placed a magnet needle next to a current carrying wire and the needle defected. This discovery indicated that there was a connection between electricity and magnetism. This lead Arago to study the magnetism, one of his first discoveries was to show that substances other than iron could be magnetic. He was the first to show that a coil of wire carrying a current could act as a magnet, this forms the basis for an electromagnet.

Arago’s Disc

He also notices that when a copper disk was rotated, a magnetic needle which was freely suspended above it would rotate with the disk. If the copper disc was reversed so was the needle. This device was called Arago’s disc and it baffled several imminent scientists including Sir John Herschel, Simeon Poisson and even Arago did not understand it. It took the genius of Faraday to explain the motion and it helped him too in his studies of electromagnetism.

The End Of Slavery

Arago was director of the Observatoire de Paris from 1843-1853 and discovered the Sun’s chromosphere. He suggested that his student Le Verrier investigate irregularities in Uranus’s orbit. Adams, claimed to have discovered Neptune first, but this is disputed by Arago who thought that Le Verrier had found the planet. Arago was a firm republican and was minister of war and marine in the provisional government which took power after the 1848 Revolution. One of his many accomplishments was abolishing slavery in the French colonies.

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