FIRST CONFIRMATION OF MAXWELL'S ELECTROMAGNETIC THEORY

LEBEDEV, P.N.

Untersuchungen über die Druckkräfte des Lichtes.

Leipzig, Ambrosius Barth, 1901. 8vo. Bound in a nice contemporary half calf with red leather title label and gilt lettering to spine. In "Annalen der Physik", Vol. 6, 1901. Entire volume offered. Stamp to lower part of title-page. Upper front hindge with 3 cm long tear. otherwise a fine copy. Pp. 433-58. [Entire volume: VIII, 876 pp. + 3 folded plates].

First appearance of the account of Lebedev's seminal experiment proving that light exerts a mechanical pressure on material bodies thereby confirming Maxwell's electromagnetic theory for the very first time - a landmark discovery in modern physics.

James Clerk Maxwell "made an important new prediction from his electromagnetic theory-that electromagnetic waves exert a radiation pressure. Bright sunlight, he calculated, presses on the earth's surface with a force of around 4 pounds per square mile [...] This was too tiny a value to be observable in everyday life and its detection posed a challenge to experimenters. Eventually, in 1900, the Russian physicist Pyotr Lebedev succeeded, and confirmed James' prediction. Although small on an earthly scale, radiation pressure is one of the factors that shape the universe. Without it there would be no stars like our sun. [The] discovery also helped to explain a phenomenon that had puzzled astronomers for centuries-why comets' tails point away from the sun" (Mahan, The Man who Changed Everything: The Life of James Clerk Maxwell, 183).

"As early as 1891 Lebedev became seriously interested in the pressure of light. He turned his attention to the fact that since the force of gravity is proportional to the volume of a body whereas light pressure must be proportional to its surface, it may be asserted that in a particle of cosmic dust the forces of light pressure pushing the particle away from the sun will be equals to the force of gravity attracting it toward the sun. Lebedev used this theory to explain why comets'tails always point away from the sun. His hypothesis was considered correct until the discovery of the solar wind, which creates substantially greater pressure than the sun's light.
Around 1898, Lebedev began experimental research on light pressure. Although its presence had been predicted by Maxwell's theory, it had not been detected experimentally before Lebedev. He first undertook research on the pressure of light on solid bodies. Because of the weakness of the effect itself and the considerable number of possible side effects, this experimental problem presented very great difficulties: if a body that is supposed to react to light pressure is placed in a gas, the warming of the body by the light will inevitably cause convection currents and thus set the body in motion. If the body is placed in a vacuum (in practice, in gas at very low pressure), the so called radiometric effect will occur. As a result of the uneven warming of the front and back of the body, the molecules of gas hitting the body from the front will be repulsed more forcefully than those striking the back, thereby exerting greater pressure. By extremely ingenious methods Lebedev succeeded in completely eliminating these side effects and not only detected the pressure of light but also measured it and showed the correctness of Maxwell's quantitative theory. "Opytnoe issledovanie svetovogo davlenia"("An Experimental Investigation of the Pressure of Light") was read by Lebedev at the International Congress of Physicists at Paris in 1899 and was published in 1901." (DSB)

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