Jean Baptiste Biot

views updated

Jean Baptiste Biot


French Physicist

Jean Baptiste Biot is perhaps best described as a polymath who made important contributions to acoustics, optics, and electromagnetic theory during a career that also included significant work in astronomy, geodesy, and many other fields. In 1803 he helped confirm that meteorites are of extraterrestrial origin, and in 1804 made a balloon ascent with French physicist and chemist Joseph Gay-Lussac (1778-1850). In 1856, in recognition of his writings on several subjects, Biot was elected to the Academie Française, an honor quite rare for a scientist. A dominant figure within the French university system, he was among the first to recognize and encourage the young Louis Pasteur (1822-1895) in his scientific career.

Biot was born the son of a treasury official, the first in his family to rise from the peasantry to a position of social standing. He served in the army during the French revolutionary period and his subsequent career reflected both the ideals of the revolution and the new institutions introduced in France at that time. He was among the first students to enter the Ecole Polytechnique and, following graduation and some minor academic posts, became Professor of Astronomy at the Collège de France when it was established by Napoleon Bonaparte in 1806. Remaining at this post until his retirement in 1849, Biot enjoyed the reputation of an outstanding teacher.

News of the discovery by Danish physicist Hans Oersted (1777-1851) of a connection between electricity and magnetism reached Paris in September 1820. By October 30 of the same year, Biot and Felix Savart (1791-1841) were able to deliver a paper at a meeting of the Academie des Sciences establishing the quantitative law for the magnetic force between currents flowing in different electrical circuits, now generally known as the Biot and Savart law.

The greatest contribution made by Biot, however, lies in the area of optics, particularly the behavior of polarized light. Until 1808, when Etiennne Malus (1775-1812) discovered that light beams reflected by a smooth surface would be polarized if the reflection occurred at the appropriate angle, the only clue to the polarizability of light was the double refraction, or splitting, of light beams that occurs in crystals of a low degree of symmetry. Biot became active in the field following the 1811 discovery by his colleague Dominique Arago (1786-1853) that light polarized by reflection could be split into two differently colored beams by certain crystals. Biot, an admirer of Isaac Newton (1642-1727), explained this result in terms of forces acting on the particles of light that comprised a light beam. The following year Biot observed that the rotation of the plane of polarization produced by a plate of quartz was dependent on the color, a process now known as rotatory dispersion.

In 1815 Biot showed that the rotation of the plane of light polarization was not restricted to crystalline substances, but could be observed in certain liquids and even vapors. He concluded, correctly, that the rotation was a characteristic of the individual molecules. In 1816 he used the effect to argue for the chemical identity of sugar derived from sugar beets and sugar cane. In 1832 he began the study of another material of biological origin, tartaric acid. Four years later he published a paper devoted entirely to the rotation of polarized light by this substance and noted that it depended on color in a manner different from that of the substances he had previously investigated. Biot's investigation of optical rotation by tartaric acid and its salts was continued by Pasteur, in whose career Biot took a personal interest. In his first major research, Pasteur established the power of the optical rotation technique for future chemical research.