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‎Ersetzung der Hypothese vom unmechanishen Zwang durch eine Forderung bezüglich des inneren Verhaltens jades einzelnen Elektrons. - [DISCOVERY OF ELECTRON SPIN]‎

‎Berlin, Springer, 1925. Royal8vo. Bound in contemporary half cloth with paper label with handwritten title to spine. In ""Die Naturwissenschaften"", Vol. 30, 1925. Library stamp to title page, otherwise fine and clean. Pp. 953-954. [Entire volume: XXIII, (1), 1092, 44 pp.].‎

‎First printing of this landmark paper in which electron spin was first announced. ""Ehrenfest had suggested that he work with a fellow student, Goudsmit, who already was an expert on atomic spectra. The two of them wrote a paper on the spectrum of hydrogen, giving an improved interpretation involving half-integer quantum numbers. Then came their great discovery: electron spin. The quantum numbers they had assigned implied that the electron must have another degree of freedom"" it must be rotating. With this idea everything fell into place. They published first a short note and then, at the encouragement of Niels Bohr, who visited Leiden shortly after their discovery, a longer paper that appeared in Nature. The origin of the spin-orbit interaction was apparently suggested by Albert Einstein, who also visited Leiden at the time and pointed out that in its rest frame, the electron sees an orbiting nucleus and hence a magnetic field. There remained an awkward factor of two in the spin precession rate. This was soon explained as a relativistic effect in an elegant paper by Llewellen H. Thomas. In their third and final paper on electron spin, Goudsmit and Uhlenbeck summarized these results and gave what has become the accepted interpretation of electron spin and atomic spectra."" (DSB)The discovery of electron spin was proposed as an explanation for the Stern-Gerlach experiment. In this experiment a beam of silver atoms was allowed to pass through an inhomogeneous magnetic field. When the magnetic field was on, the beam of silver atoms split into two parts, one deflected up and the other deflected down. In 1925, Uhlenbeck and Goudsmit, both graduate students at the time, proposed an explanation for the Stern-Gerlach experiment by postulating that the electron possesses an intrinsic angular momentum, referred to as ""spin"".This intrinsic angular momentum gives rise to a magnetic moment in the electron that interacts with magnetic fields. The electron spin and the related magnetic moment are quantized such that there are only two possible discrete values. This quantization of the magnetic moment is what leads to the deflection of the beam of silver atoms either up or down in the inhomogeneous magnetic field. Their insight furnished a missing link leading to the final triumph of the then-struggling birth of quantum mechanics.‎