Becquerel, Alexandre-Edmond

Becquerel, Alexandre-Edmond

(b. Paris, France, 24 March 1820; d. Paris, 11 May 1891)

experimental physics.

Becquerel was the second son of Antoine-César Becquerel, experimental physicist and professor at the Muséum d’Histoire Naturelle. At the age of eighteen, Edmond was admitted to both the École Polytechnique and the École Normale Supérieure. He decided, however, to refuse both these opportunities in order to assist his father at the museum. After having served as assistant at the University of Paris and then as professor of physics at the short-lived Institut Agronomique de Versailles, Becquerel was appointed, in December 1852, to the chair of physics at the Conservatoire des Arts et Métiers. From 1860 to 1863, he taught chemistry at the Société Chimique de Paris; and after having served a term as aide-naturaliste at the museum, he succeeded his father as director of that institution in 1878. Becquerel received a Doctor of Science degree from the University of Paris in 1840 and was elected to the Académie des Sciences in 1863.

Becquerel’s most important achievements in science were in electricity, magnetism, and optics. In electricity he measured the properties of currents and investigated the conditions under which they arose. In 1843 he showed that Joule’s law governing the production of heat in the passage of an electrical current applied to liquids as well as to solids. In 1844 he rectified Faraday’s law of electrochemical decomposition to include several phenomena that had not been taken into account, and in 1855 he discovered that the mere displacement of a metallic conductor in a liquid was sufficient to produce a current of electricity. Becquerel’s measurement of the electromotive force of the voltaic pile was achieved through the use of his father’s ingeniously devised electrostatic balance, an instrument that, in effect, allowed him to weigh the relative force of an electric current. He also studied the separate effects of the liquid, the metal, the temperature, and the polarization of the electrodes on the functioning of voltaic piles.

From 1845 to 1855, Becquerel devoted most of his attention to the investigation of diamagnetism. Anxious to preserve the simplicity of Ampere’s electrical theory of magnetic action, he was unwilling to accept Faraday’s contention that diamagnetic phenomena were fundamentally different from those of ordinary magnetism. To explain the repulsion of certain substances by the poles of a magnet, he conceived an “Archimedean law” of magnetic action, so called because of its resemblance to Archimedes’ hydrostatic principle: “A body placed away from a magnetic center is attracted toward that center with a force equal to the difference which exists between the specific magnetism of the body and that of the milieu in which it is immersed” (Traité d’électricité..., III, 52). Just as specifically heavier bodies sink and lighter bodies rise in a liquid, so substances less magnetic than their surroundings are pushed away from a magnet, whereas the more magnetic ones are attracted to it.

Becquerel set out to measure the magnetic properties of oxygen, one of the substances composing the milieu in which magnetic action most often took place. He condensed a large volume of the gas in a glass tube filled with highly absorbent charcoal. When placed in a magnetic field, the tube containing oxygen was found to be much more magnetic than one containing only charcoal. This discovery might have provided conclusive proof of the validity of Becquerel’s theory had it not been for the highly embarrassing fact that diamagnetic action took place in the vacuum as well as in the air. Becquerel attempted in vain to overcome this objection by assigning magnetic (and thus electrical) properties to the ether.

Becquerel’s early investigations of light phenomena were closely related to his interest in electricity. In 1840 he demonstrated that electrical currents arose from certain light-induced chemical reactions. On the basis of this discovery, he constructed an instrument, called an “actinometer,” that measured the intensity of light by measuring the intensity of the electrical currents produced in photochemical reactions. Later, he showed that by relating light intensity to heat intensity one could use this device to determine optically the temperatures of extremely hot bodies.

In spectroscopy Becquerel revealed in 1843 the presence of Fraunhofer lines in photographs of the ultraviolet portion of the spectrum. Earlier he had shown that rays at the red end of the spectrum reinforced or “continued” the chemical action initiated by rays at the violet end.

Becquerel did his most important work in optics on the phenomena of luminescence. In the middle years of the nineteenth century, he virtually monopolized the significant discoveries made in this field. His researches began in 1839, when he published a paper dealing primarily with the effects of temperature on the duration of phosphorescent light emission. In 1843 he demonstrated that phosphorescence was stimulated in different substances by specific frequencies of light and that at some frequencies the phosphorescent glow seemed to stop immediately after the cutting off of incident light rays. Between 1857 and 1859, Becquerel produced three pioneering studies on luminescent phenomena; these were collected and published in 1859 under the title Recherches sur les divers effets lumineux qui resultent de l’action de la lumiére sur les corps. It was in these studies that Becquerel first described the phosphoroscope, an instrument of his own invention consisting of a box sealed with two disks mounted on the same axis and pierced with holes arranged in such a way that light could not at any one time pass through the entire apparatus. By rapidly revolving these perforated disks, an observer could continuously view substances in the dark only fractions of a second after they had been exposed to brilliant light; and by regulating the speed of revolution of the disks, one could measure the length of time that substances continued to glow after exposure to light. Using this device, Becquerel was able to identify many new phosphorescent substances and to show that the phenomenon G. G. Stokes had named fluorescence in 1852 was in reality only phosphorescence of an extremely short duration. By attaching a prism to his phosphoroscope, Becquerel was able to examine the spectra of light emitted from luminescent bodies. In this manner substances could be analyzed without physical or chemical alteration.

BIBLIOGRAPHY

I. Original Works. Becquerel’s Traité d’électricité et de magnétisme, leurs applications aux sciences physiques, aux arts et à l’industrie, 3 vols. (Paris, 1855–1856), was written in collaboration with his father. Besides the Recherches sur les divers effets lumineux... (Paris, 1859), Becquerel wrote La humiére, ses causes et ses efferts, 2 vols. (Paris, 1867–1868). The first volume of this work is devoted to the study of luminescent phenomena.

II. Secondary Literature. Works on Becquerel are Henri Becquerel, “La chaire de physique du Muséum,” in Revue scientifique, 49 , No. 22 (28 May 1892), 673–678; and Jules Violle, “L’oeuvre scientifique de M. Edmond Becquerel,” ibid., No. 12 (19 March 1892), 353–360. See also E. N. Harvey, A History of Luminescence From the Earliest Times Until 1900 (Philadelphia, 1957), 207–219, 351–360.

J. B. Gough