A.D. 1820.—Arago (Dominique François Jean), famous French astronomer, physicist and statesman (1786–1853), who at the early age of twenty-three had, besides being Assistant Astronomer to the Observatory, become the successor both of Lalande in the Academy of Sciences and of Monge in the chair of analytical mathematics at the Polytechnic School, and who, conjointly with Gay-Lussac, had founded the highly valued Annales de Chimie et de Physique in 1816, communicates to the French Institute, on the 25th of September 1820, his discovery that the electric current has the power of developing magnetism in iron and steel. Into the axis of a galvanic conductor made in the form of a coil, or helix, he placed a needle, the extremities of the wire coil being connected to the poles of a battery, and with this he proved that the wire not only acted on bodies already magnetized, but that it could develop magnetism in such as did not already possess the power. When soft iron was used, the magnetism given was only temporary, but on repeating the experiment, M. Arago succeeded completely in permanently magnetizing small steel needles. Arago’s paper on the subject appears at p. 94, Vol. XV of the Ann. de Ch. et de Ph., and it is said that at about the same time Dr. Thos. J. Seebeck (1770–1831), and Georg Friedrich Pohl (1788–1849) laid similar results before the Berlin Academy, also that Sir Humphry Davy independently made a like discovery, of which he advised Dr. Wollaston, Nov. 12, 1820. Reference to this fact has already been made at Davy, under date A.D. 1801, wherein it was stated that the latter had found iron filings to so adhere to the connecting wire as to form a mass ten or twelve times the thickness of the wire. This was also the case in the experiments of M. Arago, who, upon observing that the filings rose before coming in contact with the conjugate wire, drew the conclusion that each small piece of iron was converted into a temporary magnet. Thus was Arago led to the discovery of what is called magnetic induction by electric currents, or, in other words, that an electrical current passing through a conductor will induce magnetic action in such bodies near it as are capable of being magnetized (Phil. Trans. for 1821, p. 9; Tilloch’s Jour. of Sci., Vol. LVII. p. 42, 1821; eighth “Britannica,” Vol. VIII. p. 532 and Vol. XIV. p. 640; Thomas Thomson, “Outline of the Sciences,” p. 563).
A fact worth noting in connection with the development of Oersted’s discovery by both Arago and Ampère, is that in order “to prevent the communication of the electricity laterally in the folds of the coil, the wire was insulated by varnish in the first instance and afterward by winding silk or cotton around it” (F. C. Bakewell, “Elec. Sci.,” London, 1853, p. 37).
On the 22nd of November 1824, Arago announced to the French Academy of Sciences the remarkable discovery made by him of a new source of magnetism in rotatory motion. He was led to this by observing that when a magnetic needle was oscillating above or close by any body, such as water or a plate of metal, it gradually oscillated in arcs of less and less amplitude, as if it were standing in a resisting medium, and, besides, that the oscillations performed in a given time were the same in number (Humboldt’s “Cosmos,” “Magnetic Observations,” 1825). He caused a circular copper plate to revolve immediately beneath a magnetic needle or magnet, freely suspended so that the latter might rotate in a plane parallel to that of the copper plate, and he found that the needle tends to follow the circumvolution of the plate; that it will deviate from its true direction, and that by increasing the velocity of the plate the deviation will increase till the needle passes the opposite point, when it will continue to revolve, and at last with such rapidity that the eye will be unable to distinguish it. This, says Mrs. Somerville, is quite independent of the motion of the air, since it is the same if a pane of glass be interposed between the magnet and the copper. When the magnet and the plate are at rest, not the smallest effect, attractive, repulsive, or of any kind, can be perceived between them. In describing this phenomenon Arago states that it takes place not only with metals, but with all substances, although the intensity depends upon the kind of substance in motion.
Arago’s experiments were repeated in London, March 7, 1825. His valuable discovery, which obtained for him the Copley medal, and which confirms the doctrine of the universal prevalence of magnetism in all bodies, is recorded in Arago’s “Sur les Déviations ... aiguille aimantée” (An. de Ch. et de Ph., Vol. XXXIII, and Phil. Trans., p. 467 for 1825), and a solution of the phenomena is given by Faraday in Phil. Trans. for 1832, p. 146, by Sir John Leslie in the Fifth Dissertation of the eighth “Britannica,” p. 746, as well as in the article “Magnetism” of the latter publication, and in Mrs. Somerville’s “Conn. of Phys. Sc.,” pp. 325–327. (See also the observations recorded in Humboldt’s “Cosmos,” 1849, Vol. I. pp. 172, 173; in Dr. Thomson’s “Outline of the Sciences,” pp. 556–558; Fahie, pp. 282, 283, 321; Dr. Whewell, Vol. II. pp. 254–256; Brewster’s Edin. Jour. of Sci., 1826, Vol. III. p. 179; “Dict. Gén. de Biogr. et d’Histoire,” Paris, 2e ed. p. 126.)
In Brewster’s Edinburgh Journal of Science (Vol. V. p. 325), notice is given of Arago’s then recent researches on the influence which bodies considered not magnetic have on the motions of the magnetic needle, and reference is made to a new communication transmitted by Arago to the Académie des Sciences, as well as to a report of additional experiments in the same line given at meetings held July 3 and 10, 1826. Arago satisfactorily meets the denials made by Leopoldo Nobili and another Italian natural philosopher (Liberato Giovanni Bacelli) that substances not metallic have any influence on the magnetic oscillations, and he announces as a result of his investigations that, for certain positions of a vertical needle, and for velocities of rotation sufficiently rapid, the repulsive force which is exerted in the direction of the radius is as great as the force perpendicular to the radius, of which the effects are observed upon a horizontal needle.
Poisson having stated in his memoir “On the Theory of Magnetism” in motion (see Poisson at A.D. 1811) that Coulomb had recognized the magnetic virtue in all bodies, independently of the iron which they contain, Arago remarked that the idea of Coulomb was quite different from his, Coulomb having been of opinion that a quantity of iron, although too small for chemical analysis even to appreciate, was sufficient to produce in bodies which contained it appreciable magnetic effects. MM. Thénard and La Place confirmed this remark. Brewster adds that, in justice to Coulomb, it is necessary to state that he is the undoubted author of the discovery that all bodies, whether organic or inorganic, are sensible to the influence of magnetism. M. Biot has remarked that there are two ways of explaining this, either all substances in nature are susceptible of magnetism, or they all contain portions of iron, or other magnetic metals, which communicate to them this property. This last explanation, though adopted by Coulomb, by no means affects his claim to the discovery of the general fact that all bodies, whether organic or inorganic, are susceptible of becoming magnetic. Prof. Hansteen has drawn from numerous experiments and observations the important conclusion that every vertical object, of whatever material it is composed, has a magnetic south pole above, and a north pole below (Edin. Phil. Journal for January-April 1821).
M. Arago made many valuable investigations concerning the influence of the aurora borealis on the needle, on the variations of the latter, upon the nature of meteors, lightning, the zodiacal light, magnetic storms, etc. etc., which are admirably recorded more particularly in the great work of Alex. von Humboldt. The latter remarks that Arago has left behind him a treasury of magnetical observations (upward of 52,600 in number) carried on from 1818 to 1835, which have been carefully edited by M. Fédor Thoman, and published in the “Œuvres Complètes de François Arago” (Vol. IV. p. 493). Much could be said, especially regarding Arago’s paper, presented by him to the Academy of Sciences in 1811, which is considered to have established the foundation of chromatic polarization. Mention must at any rate be made of the fact that in Humboldt’s estimation the discovery of the two kinds of polarization of light may be considered the most brilliant of the century. They, unquestionably, rank among the most splendid of optical phenomena.
Etienne Louis Malus, a distinguished French philosopher (Fifth Dissert. of “Encycl. Brit.”), discovered in 1808 polarization by reflection from polished surfaces, and Arago, during 1811, made the discovery of coloured polarization. A world of wonder, remarks Humboldt, composed of manifold modified waves of light having new properties was now revealed. A ray of light which reaches our eyes, after traversing millions of miles from the remotest regions of heaven, announces of itself in Arago’s polariscope (consisting of a plate of quartz cut across the axis placed in one end of a tube, at the other end of which is a doubly refracting prism) whether it is reflected or refracted, whether it emanates from a solid or fluid, or gaseous body, even announcing the degree of its intensity (Delambre, “Histoire de l’Astronomie,” p. 652; Humboldt, “Cosmos,” 1849, Vol. I. p. 33; Vol. II. p. 715).
In 1818, Arago was elected a F.R.S.; he became a member of the Royal Astronomical Society and also member of the Bureau des Longitudes during 1822, was made Perpetual Secretary of the Academy and Director of the Paris Observatory eight years later, and received the Rumford medal in 1850. The Copley medal given him in 1825 had never before been conferred upon a Frenchman of science. It was upon his urgent request that the “Annuaire du Bureau des Longitudes” and “Les Comptes Rendus hebdomadaires” were commenced by the Academy, 1828–1835.
In a letter to Schumacher, Humboldt speaks of Arago as “one gifted with the noblest of natures, equally distinguished for intellectual power and for moral excellence.” In conjunction with Gay-Lussac, Arago was, for almost half a century, Humboldt’s most intimate friend, and their ever-increasing intimacy became such as to lead to a perfect unity of thought on scientific subjects. It cannot, therefore, be considered an exaggerated expression of feeling when, in a letter to Geoffroy St. Hilaire, dated Berlin, June 24, 1829, Humboldt should conclude with the words: “Pray remember me to MM. Valenciennes, Deleuze and Cuvier, but especially to him whom I hold dearest in this life, to M. Arago.”