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Dominique Francois Jean Arago is known for his contributions to physics and astronomy. In physics, his principle work was in electromagnetism and in light. In 1820 he found that iron placed in a wire coil could be magnetized by the passage of current from either a Leyden jar or a voltaic cell. Seems that Arago was the first man to build consciounsly an electromagnet in September 1820.
In 1824 he discovered the phenomenon of magnetic rotation, in which a rotating copper disk deflects a magnetic needle suspended above it. Arago’s research in light commenced with his discovery of chromatic polarization in 1811. As an astronomer, Arago is remembered as the discoverer of the solar chromosphere and for his accurate measurements of the diameters of the planets.
Dominique Francois Jean Arago was born at Estagel a small village near Perpignan, in the d"partement of Pyrenees-Orientales, France, on February 26, 1786. He was the eldest of four brothers (three other brothers: Jean, Jacques, and Etienne). Jean Arago (1788-1836) emigrated to North America and became a general in the Mexican army. Jacques Etienne Victor Arago (1799-1855) took part in de Freycinet’s exploring voyage in the Uranie from 1817 to 1821, and on his return to France devoted himself to his journalism and the drama. The fourth brother, Etienne Vincent de Arago (1802-1892), is said to have collaborated with Honore de Balzac in The Heiress of Birague, and from 1822 to 1847 wrote a great number of dramatic pieces, mostly in collaboration.
Native house of Arago in Estagell
Arago was educated first at Perpignan, then at the Ecole Polytechnique (Polytechnic school) in Paris. It is known from his autobiography that even the professors in the Polytechnic school were very distinguished, they were remarkably incapable of imparting their knowledge or maintaining discipline. In 1804 Arago upon recommendation of Simeon Poisson was made secretary to the observatory at Paris.
Then from 1806 to 1809 Arago was in Spain, where he accompanied Jean Baptiste Biot on a measurement of an arc of meridian of the Earth which led to the standardisation of the metric system of lengths. On his return to Paris in 1809 he was elected to the Acad"mie des Sciences and received the chair of analytical geometry at the Ecole Polytechnique, Paris, where, at the age of 23, he succeeded Gaspard Monge. In 1830 he succeeded J.B.J. Fourier as the permanent secretary of the Ecole Polytechnique. He also was director of the Paris Observatory. He enjoyed a marked success as a lecturer.
He gave popular lectures on astronomy, which were both lucid and accurate - a combination of qualities which was rarer then than now. He reorganized the national observatory, the management of which has long been inefficient, but in doing this his want of tact and courtesy raised many unnecessary difficulties. Arago worked in a number of branches of physics.
Francois Arago
His first investigations concerned the polarization of light and in 1811 he discovered chromatic polarization. Working with Fresnel he discovered that two beams of light polarised in perpendicular directions do not interfere, leading to the transverse theory of light waves. Thus, Arago supported A.-J. Fresnel’s wave theory of light against the emission theory favoured by P.-S. Laplace, J.-B. Biot, and S.-D. Poisson.
According to the wave theory, light should be retarded as it passes from a rarer to a denser medium; according to the emission theory, it should be accelerated. Arago’s test for comparing the velocity of light in air and in water or glass was described in 1838, but the experiment required such elaborate preparation that Arago was not ready to perform until 1850, when his sight failed. Before his death, however, the retardation of light in denser media was demonstrated by A.-H.-L. Fizeau and L"on Foucault, who used his method with improvements in detail. This was the triumph of the wave theory of light.
In 1820, elaborating on the work of H.C. Orsted of Denmark, 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 and that the filings fell off when the current ceased. Seems that Arago was the first man to build consciounsly an electromagnet, in September 1820, in the same time he noticed that rhe iron became a magnet only during the flow of current, the effect finished when the current stopped: the basis of Morse telegraph.
Arago had also discovered, in 1824, that a disk of non-magnetic metal had the power of bringing a vibrating magnetic needle suspended over it rapidly to rest; and that on causing the disk to rotate the magnetic needle rotated along with it. When both were quiescent, there was not the slightest measurable attraction or repulsion exerted between the needle and the disk; still when in motion the disk was competent to drag after it, not only a light needle, but a heavy magnet.
The question had been probed and investigated with admirable skill both by Arago and Ampere, and Poisson had published a theoretic memoir on the subject; but no cause could be assigned for so extraordinary an action. It had also been examined by two celebrated men, Mr. Babbage and Sir John Herschel; but it still remained a mystery. Faraday saw mentally the rotating disk, under the operation of the magnet, flooded with his induced currents, and from the known laws of interaction between currents and magnets he hoped to deduce the motion observed by Arago. Faraday later proved these to be induction phenomena.
Arago’s Disk
In this device, a copper disk is rotated rapidly with a hand crank and a step-up pulley system. Balanced on a pivot above the center of the disk is a compass needle. The motion of the needle relative to the highly conducting copper disk induces eddy currents in the disk.
In turn, these eddy currents produce a torque on the magnetic needle, which starts to rotate. The presence of eddy currents may be inferred from the fact that a copper disk with radial slots cut in it produces little effect; the slots interrupt the eddy currents. The inverse effect also occurs: a spinning bar magnet will cause a suspended copper disk to rotate.
“Some years before Faraday discovered the principles of induced emf’s, Arago made a discovery which was not understood at the time. A horizontal disk of copper was mounted so that it could be set in rotation about a vertical axis. If a horizontal bar magnet was suspended over this disk, the bar wouldbe set in rotation in whichever direction the copper was turning. By some unseen force the magnet was dragged around by the moving plate. …It was not until after the discovery of induced emf’s that Arago’s experiment was understood.” From Physics, Third Edition, by O.M. Stewart, Ginn, 1939.
Observatoire de Paris
Coupole Arago
In astronomy, Arago discovered the Sun’s chromosphere and made accurate measurements of the diameters of the planets. However, Arago is best known for his part in the dispute between U.-J.-J. Le Verrier, who was his prot"g", and the English astronomer John C. Adams over priority in discovering the planet Neptune and over the naming of the planet. Arago had suggested in 1845 that his student Le Verrier investigate anomalies in the motion of Uranus. When the investigation resulted in Le Verrier’s discovery of Neptune, Arago proposed that the newly found planet be named for Le Verrier.
In Paris you can still see ‘Arago’ markers: the line of 135 little bronze disks set out along the M"ridien de Paris - which is the world’s old zero longitude, but is now 2degree20′14″ east of the new zero at Greenwich, set in 1884. France and Ireland didn’t adopt the new ‘zero’ until 1911. The disks are about 10 to 15 centimetres in diametre. Besides the name ‘Arago’ in raised engraving on them, there are also an ‘N’ and a ‘S.’
Arago’s portraits
Engraving by H.I. Torlet
Engraving by Charles Phillipe Auguste Carey, 1824-1897
Lithograph by Francois-Seraphin Delpech, 1778-1825, from the original engraving of Maguire
Printed by Alexandre Vincent Sixdeniers, 1795-1846, from the original engraving Nicolas-Eustache Maurin, 1799-1850.
Printed by Francois-Seraphin Delpech, 1778-1825, from the original engraving by Antoine Maurin, 1793-1860.
Arago played a rather dubious role in the history of photography: he championed the system of his friend, Daguerre, and apparently used his influence to persuade Hippolyte Bayard, another inventor who actually exhibited earlier than Daguerre, to keep his process secret so that the French Academy bought the rights to Daguerre’s invention.
Political career of Arago started in 1830 when he was elected deputy from Pyr"n"es-Orientales and later from Paris. Known for his republican views, Arago became a member of the provisional government formed after the Revolution of 1848. As minister of war and navy, he appointed greatest advocate of ending slavery Victor Schoelcher as undersecretary for the navy, who prepared the famous decree that abolished slavery in the colonies. Arago was elected to the National Constituent Assembly and then to the Executive Power Commission on May 10, 1848. He became president of the commission as he obtained the majority of votes at election and was the eldest member.
Portrait of Francois Arago by Ary Scheffer
Arago was a good friend of Alexander von Humboldt and Delacroix, and corresponded with Prosper Merim"e. He remained to the end a consistent republican, and after the coup d’"tat of 1852, though half blind and dying, he resigned his post as astronomer rather than take the oath of allegiance. It is to the credit of Napoleon III that he gave directions that the old man should be in no way disturbed, and should be left free to say and do what he liked. However, soon after that, Arago died in Paris on October 2, 1853.
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