Becquerel, Alexandre-Edmond (1820-1891)

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BECQUEREL, ALEXANDRE-EDMOND (1820–1891)

Edmond Becquerel was one of a family of scientists. His father, Antoine-César, was professor of physics at the Muséum d'Histoire Naturelle, and his son, [Antoine-]Henri Becquerel, also a physicist, discovered the phenomenon of radioactivity (for which he received the Nobel Prize in 1903).

The scientific work of Edmond began in 1838, at the very early age of eighteen. When the Chair of Professor of Physics Applied to Natural History was created for his father at the Muséum d'Histoire Naturelle, Edmund had the dilemma of choosing to attend l'Ecole Normale, l'Ecole Polytechnique, or become an assistant to his father for the course that went with the professorship. He chose to assist his father, and their collaboration continued for decades. Thus, his title on the title page of the book published in 1855-1856 with his father is given as: "Professeur au Conservatoire impérial des Arts et Métiers, Aide-naturaliste au Muséum d'Histoire Naturelle, etc." After a short period as assistant at la Sorbonne, and then as Professor at the Institut Agronomique de Versailles, he became Professor at the Conservatoire des Arts et Métiers in 1852, where he worked for almost forty years. When his father died in 1878, Edmond succeeded him as director of the Muséum in addition to his professorship. He received a degree as Doctor of Science from the University of Paris in 1840, and was elected a member of l'Académie des Sciences in 1863.

He published a great number of scientific articles and a number of books: the three volume Traité d'électricité et de magnétisme, et des applications de ces sciences á la chimie, á la physiologie et aux arts. (1855-1856 with his father); Recherches sur divers effets lumineux qui résultent de l'action de la lumiére sur les corps(1859); and La lumiére, ses causes et ses effets1867, in two volumes.

Electricity, magnetism, and light were the main subjects of his work. At the time, these subjects were "hot" topics. Hans Christian Ørsted had made his discovery that an electric current had an effect on a magnet in the year Edmond was born. Michael Faraday had just (in 1831) discovered the effect of induction, and Louis Daguerre invented the photographic plate in 1837, the year before Edmund began his scientific work. Edmond set out to study the chemical effect of light, and in 1839 he discovered a remarkable effect: electricity was emitted following the chemical actions due to the light—the photoelectric effect. He was thus led to the construction of the "actinometer," which allows the measurement of light intensities by measuring the electric current generated by the light. Using photographic plates, he examined the sunlight spectrum and discovered that the dark lines, observed by Fraunhofer in the visible part, continue into the violet and ultraviolet region, and that the plates, when exposed briefly to ultraviolet radiation, become sensitive to the red part as well, and can actually acquire an image without development of the plates.

Parallel to these investigations, he continued (with his father) to study electricity. He used the method of compensation to measure the resistivity of a large number of materials, including liquid solutions. The effect of the electrodes was, in the latter case, taken into account by using tubes in which it was possible to change the distance between the electrodes. Electrochemical effects and their practical applications were also a main concern. The second volume of the "Traité ..." is mainly concerned with the feasibility of extracting silver from minerals in Mexico by electrochemical methods as opposed to the methods then in use involving either charcoal or mercury. Mercury was expensive and charcoal was becoming increasingly expensive due to the shortage of wood. In the introduction to the "Traité ..." he mentions that if the consumption of wood in Mexico continued at the ten current rate, it would have severe effects, and that the Mexican government should be concerned. (It is at this point worth noting that Mexico had obtained independence in 1821, and the subject of French intervention in the internal wars that followed was a major political issue.) He studied extensively the electromotive force and internal resistance of a large number of batteries. He and his father used the thermoelectric effect to construct thermometers that could measure temperatures that were otherwise difficult to measure and at places that were hard to access by other means. The temperature in the ground was, for instance, measured throughout the year.

Another study was begun, in 1839, with Jean-Baptiste Biot (who had measured quantitatively the force that an electric current produces on a magnet, the effect that Ørsted had discovered qualitatively), namely, on phosphorescence, fluorescence and luminescence. To study the phenomenon that certain substances emit light after having been exposed to light, Becquerel devised an ingenious apparatus. The main idea was to have two discs with holes in them rotating about an axis parallel to the beam of light illuminating the sample placed between the discs. The sample receives light only when a hole passes in front of it; otherwise, the disc blocks the path of light. Likewise, the emitted light is observed only when a hole in the other disc passes the sample and can be examined at varying times after the exposure by changing either the relative positions of the holes or by changing the speed of rotation. Furthermore, a prism could be inserted in the path of the emitted light and spectral analysis performed. With this simple apparatus he was able to reduce to 1/40,000 of a second the time separating the luminous excitation and the observation. A number of important results were obtained; for instance, that fluorescence differed from phosphorescence only by its very short duration, and that the spectrum of the fluorescent light is characteristic for each substance (one of the first instances of nondestructive testing).

The discovery and detailed investigations of the phenomenon of fluorescence is generally considered the main contribution of Edmond Becquerel. It had the further impact of leading later to the discovery of radioactivity by his son Henri, as Henri continued these studies, including among the substances examined salts of uranium.

Edmond Becquerel was interested in and dedicated to science in general. He was a very careful and imaginative experimenter with an acute sense of the practical aspects of science. He put great effort and insight into exploring the practical uses of physics, especially the new phenomena of electricity and magnetism or, when combined, electromagnetism.

Stig Steenstrup