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Summary Article: Arago, (Dominique) François (Jean) (1786-1853)
From The Hutchinson Dictionary of Scientific Biography

Place: France

Subject: biography, physics

French scientist who made contributions to the development of many areas of physics and astronomy, the breadth of his work compensating for the absence of a single product of truly outstanding quality. He was closely involved with André Ampère in the development of electromagnetism and with Augustin Fresnel in the establishment of the wave theory of light. Arago's political commitment demanded much time during his latter years, but he maintained a continuous flow of scientific investigations until almost the end of his life.

Arago was born in Estagel on 26 February 1786. He studied at the Ecole Polytechnique in Paris and was then appointed to the Bureau of Longitudes. He travelled to the south of France and Spain with Jean Biot (1774-1862) in 1806, where they intended to measure an arc of the terrestrial meridian. Biot returned to France in 1807, but Arago continued his work amidst a deteriorating political situation. His return to France in 1809 was somewhat enlivened by a shipwreck and his subsequent near-escape from being sold into slavery in Algiers.

In the same year, Arago was elected to the membership of the French Academy of Sciences and became professor of analytical geometry at the Ecole Polytechnique, a post he held until 1830. He became a Fellow of the Royal Society of London in 1818, which awarded him the Copley Medal in 1825 for his work on electromagnetism.

The year 1830 was one of several changes for Arago. He resigned his post at the Ecole Polytechnique and succeeded Jean Fourier as permanent secretary to the Academy of Sciences. He also became director of the Paris Observatory and deputy for Pyrénées Orientales, a commitment he retained until 1852.

Arago's political affiliation was with the extreme left, and the political turbulence of 1848 saw him elected to a ministerial position in the provisional government. It was under his administration that slavery was abolished in the French colonies. He resigned his post of astronomer in 1852 upon the coronation of Emperor Napoleon III, refusing to take an oath of allegiance to the emperor. Arago's reputation protected him but he died soon afterwards in Paris on 2 October 1853.

Arago's one area of sustained scientific effort was the study of the nature of light. The controversy over the question of whether light behaves as a stream of particles or as a wave motion was one of the most hotly debated of the time. Arago initially sided with Biot in the particulate camp, but later took the other view with Baron Humboldt, Augustin, Fresnel, and others. In 1811, he invented the polariscope, with which he was able to measure the degree of polarization of light rays. From 1815, Arago worked with Fresnel on polarization and was able to elucidate the fundamental laws governing it. Fresnel's mathematical expertise complemented Arago's experimental ability in establishing the wave theory of light, though, because of difficulties in explaining its transmission through the aether, Arago could not accept Fresnel's assertion that light moves in transverse waves.

In 1838, Arago published a method for determining the speed of light using a rotating mirror. He was interested to find out how the speed of light is affected by travelling through a medium such as water that is dense in comparison with air. The experiment was a crucial one in determining whether light is a wave motion or not, but sadly difficulties with his laboratory equipment, the 1848 revolution, and finally the loss of his eyesight in 1850 prevented Arago from completing the experiment. Léon Foucault was able to do this for him, and he obtained results which confirmed the wave theory before Arago's death.

In 1820, Arago announced to the Academy of Sciences some observations on the effect of an electric current on a magnet that had been obtained by Danish physicist Hans Oersted. Arago himself turned to the study of electromagnetism, inspiring Ampère to do the same and producing many interesting results himself. Arago found in 1820 that an electric current produces temporary magnetization in iron, a discovery crucial to the later development of electromagnets and electric relays and loudspeakers. Then, in 1824, he discovered that a rotating nonmagnetic metal disc - for example, of copper - deflects a magnetic needle placed above it. This was a demonstration of electromagnetic induction, explained by Michael Faraday in 1831.

Arago also investigated the compressibility, density, diffraction, and dispersion of gases; the speed of sound, which he found to be 331.2 m/1,087 ft per second; lightning, of which he found four different types; and heat. His studies in astronomy included investigations of the solar corona and chromosphere, measurements of the diameters of the planets, and a theory that light interference is responsible for the ‘twinkling’ of stars.

Arago's energy and enthusiasm, while directing him into a multiplicity of endeavours, acted as a catalyst in the achievement of several fundamental advances in the study of light and electromagnetism.

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