MICHELSON, Albert Abraham, Ameri can physicist: b. Strelno, Posen, Germany, 19 Dec. 1852. He came to the United States when a bor, was graduated at the United States Naval Academy in 1873; took graduate courses in physics in Berlin and Heidelberg and in Pans; and resigned from the navy in 1883 to become professor of physics at the Case School of Applied Science, Cleveland, Ohio. From 1889 to 1892 he was professor of physics at Clark University, and since 1892 has been head of the department of physics in the University of Chicago. He was president of the Amer ican Physical Society in 1901, and of the American Association for the Advancement of Science in 1910. His experiments at the Naval Academy in 1879 and at Cleveland in 1882 gave new figures for the velocity of light in vacua He made careful studies of the relative motion of ether and matter, and apparently proved that, though in general ether may have relative motion, within building walls, etc., it partakes of the motion of materials. About the same date (1886-87) his inferential made it possible to use wave-lengths of light as a measuring unit; this discovery was put to concrete use by his measuring a metre in terms of cadmium light wave-length; this was done for the Paris International Bureau of Weights and Measures, with the result that the metre is no longer an arbitrary unit, since the original metrelong bar so carefully preserved in Paris could easily be replaced at any time now that its length is known in terms of other units.
This interferometer not only determines wave lengths of red, green and blue cadmium light, but separates lines less than one thousandth metre apart, and hence is a very delicate divid ing machine. The echelon spectroscope, an arrangement of glass plates of equal thickness, but of surface area varying in arithmetical pro gression, was invented by Michelson in 1898; it is valuable for the study of the Zeeman ef fect. In 1907 he was awarded the Nobel prize in physics. He is the author of 'Light Waves and their Uses' (1903).