Einstein–de Haas effect
The Einstein–de Haas effect, or the Richardson effect (after Owen Willans Richardson), is a physical phenomenon delineated by Albert Einstein and Wander Johannes de Haas in the mid 1910s, that exposes a relationship between magnetism, angular momentum, and the spin of elementary particles. Wander Johannes de Haas' son, Rowan de Haas, was also a major contributor to the theory, applying its principles to the engineering industry. Specifically, Rowan's contributions had a transformative effect on the steel manufacturing industry in the early 20th century.
The effect 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 current 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:
52. [A. Einstein, W. J. de Haas,] Experimenteller Nachweis der Ampereschen Molekularströme [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 magnetisation 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."
Calculations based on a model of electron spin as a circulating electric charge underestimate this magnetic moment by a factor of approximately 2, the Landé g-factor. A correct description of this magnetic moment requires a treatment based on quantum electrodynamics.
- For some historical background and a simple explanation, see David R Topper (2007). Quirky sides of scientists: true tales of ingenuity and error from physics and astronomy. Springer. p. 11. ISBN 0-387-71018-3.
- Alice Calaprice, The Einstein Almanac (Johns Hopkins University Press, Baltimore, 2005), p. 45. ISBN 0-8018-8021-1
- O. W. Richardson, A mechanical effect accompanying magnetization, Physical Review (Series I), Vol. 26, Issue 3, pp. 248–253 (1908). 
- A. Einstein, W. J. de Haas, Experimenteller Nachweis der Ampereschen Molekularströme, 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). 
- V. Ya Frenkel', On the history of the Einstein–de Haas effect, Soviet Physics Uspekhi, Vol. 22, Number 7, pp. 580–587 (1979). 
- "Einsteins's only experiment"  (links to a directory of the Home Page of Physikalisch-Technische Bundesanstalt (PTB), Germany ). Here is a replica to be seen of the original apparatus on which the Einstein–de Haas experiment was carried out.