Einstein-de Haas effect
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The Einstein-de Haas effect. The Einstein-de Haas effect (named after Albert Einstein and Wander Johannes de Haas), or the Richardson effect (after Owen Willans Richardson), corresponds to the mechanical rotation that is induced in a ferromagnetic material (of cylindrical shape and originally at rest), suspended with the aid of a thin string inside a coil, on driving an impulse of electric current through the coil. To this mechanical rotation of the ferromagnetic material (say, iron) is associated a mechanical angular momentum, which, by the law of conservation of angular momentum, must be compensated by an equally large and oppositely directed angular momentum inside the ferromagnetic material. Given the fact that an external magnetic field, here generated by driving electric current through the coil, leads to magnetisation of electron spins in the material (or to reversal of electron spins in an already magnetised ferromagnet — provided that the direction of the applied electric currect is appropriately chosen), the Einstein-de Haas effect demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics. This is remarkable, since electron spin, being quantized, cannot be described within the framework of classical mechanics.
Commenting on the papers by Einstein, Calaprice in The Einstein Almanac writes:[1]
52. [A. Einstein, W. J. de Haas,] Experimenteller Nachweis der Ampereschen Molekularstörme [Experimental Proof of Ampère's Molecular Currents], Deutsche Physikalische Gesellschaft, Verhandlungen 17 (1915): 152-170.
Considering Ampère's hypothesis that magnetism is caused by the microscopic circular motions of electric charges, the authors proposed a design to test Lorentz's theory that the rotating particles are electrons. The aim of the experiment was to measure the torque generated by a reversal of the megnetisation of an iron cylinder.
Calaprice further writes:
53. [A. Einstein, W. J. de Haas,] Experimental Proof of the Existence of Ampère's Molecular Currents (in English), Koninklijke Akademie van Wetenschappen te Amsterdam, Proceedings 18 (1915-16).
Einstein wrote three papers with Wander J. de Haas on experimental work they did together on Ampère's molecular currents, known as the Einstein-de Haas effect. He immediately wrote a correction to paper 52 (above) when Dutch physicist H. A. Lorentz pointed out an error. In addition to the two papers above [that is 52 and 53] Einstein and de Haas cowrote a "Comment" on paper 53 later in the year for the same journal. This topic was only indirectly related to Einstein's interest in physics, but, as he wrote to his friend Michele Besso, "In my old age I am developing a passion for experimentation."
It is interesting to note that an attempt to describe the magnetic moment corresponding to an electron spin in terms of a circulating electric charge leads to an underestimation of this magnetic moment by a factor of 2 (more precisely by 
, the Landé gyromagnetic factor, also known as 
-factor). A correct description of this magnetic moment requires a treatment based on the relativistic quantum theory of Dirac.
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[edit] Note
- ^ Alice Calaprice, The Einstein Almanac (Johns Hopkins University Press, Baltimore, 2005), p. 45. ISBN 0-801-88021-1
[edit] References
- O. W. Richardson, A mechanical effect accompanying magnetization, Physical Review (Series I), Vol. 26, Issue 3, pp. 248-253 (1908). [1]
- A. Einstein, W. J. de Haas, Experimenteller Nachweis der Ampereschen Molekularstörme, Deutsche Physikalische Gesellschaft, Verhandlungen 17, pp. 152-170 (1915).
- A. Einstein, W. J. de Haas, Experimental proof of the existence of Ampère's molecular currents (in English), Koninklijke Akademie van Wetenschappen te Amsterdam, Proceedings, 18 I, pp. 696-711 (1915). [2]
- V. Ya Frenkel', On the history of the Einstein-de Haas effect, Soviet Physics Uspekhi, Vol. 22, Number 7, pp. 580-587 (1979). [3]
[edit] See also
- The Barnett effect.
