Spinning Electrons and the Structure of Spectra Nature 117 No. 2938 pp. 264-265, February 20, 1926 [FIRST EDITION, ORIGINAL JOURNAL ISSUE IN ORIGINAL WRAPPERS]

FIRST EDITION, ORIGINAL WRAPS, OF THE 1ST ANNOUNCEMENT IN ENGLISH OF THE CONCEPT OF 'INTRINSIC SPIN'' OF THE ELECTRON. While still students studying at Leiden under Paul Ehrenfest, two Dutch physicists, George Uhlenbeck and Samuel Goudsmit postulated that the electron must have an intrinsic angular momentum ('spin') and therefore a magnetic moment because the electron is charged. Each electron, they argued, spins with an angular momentum of ½ Planck constant and carries a magnetic moment of one Bohr magneton" (Instituut-Lorentz, Leiden". Commentary by Bohr follows their paper. "In 1925, Pauli proposed the idea of a fourth quantum number for electrons as an attempt to explain the anomalous Zeeman effect and the Pauli exclusion principle, but did not provide a physical explanation for this new quantum number. Shortly thereafter, [the duo] introduced the concept of intrinsic electron spin to explain the angular momentum of the free electron and provide a physical explanation for the fourth quantum number that Pauli had proposed" (Wenner Collection: History of Physics). Goudsmit later wrote: "'Our luck was that the idea [of spin] arose just at the moment when we were saturated with a thorough knowledge of the structure of atomic spectra, had grasped the meaning of relativistic doublets, and just after we had arrived at the correct interpretation of the hydrogen atom' Uhlenbeck recalled: 'It was then that it occurred to me that since each quantum number corresponds to a degree of freedom of the electron, the fourth quantum number must mean that the electron had an additional degree of freedom - in other words, the electron must be rotating!'. Everything fell into place. The electron had spin ½. Landé's g-factor, g=2, does not apply to the core but to the electron itself!" (Physics Today, 1989, 39). Apart from the electron's g=2 factor there is another factor of 2 in the story of spin" (Brandt, Harvest of a Century, 151). In 1925 Uhlenbeck and Goldsmit published their findings in a short note in Naturwissenschaften - a note that caused an uproar among some physicists. Lorentz was the first to point out to Ehrenfest, Uhlenbeck, and Goldsmit "that the idea of a spinning electron would be incompatible with classical electrodynamics" (Instituut). The two physicists then asked Ehrenfest not to submit the paper, to which Ehrenfest replied that he had already sent it off, [adding famously]: "You are both young enough to be able to afford a stupidity!" (ibid). Following publication of their note, Pauli initially "rejected their theory of electron spin. stating that their calculation of the effect of intrinsic spin on the fine structure of the hydrogen atom was off by a factor of 2" (Wenner). As well, "Goudsmit got a letter from Heisenberg with congratulations on the 'brave note' and the question 'how you got rid of the factor 2?'. Heisenberg computed quantitatively the splitting of the energy levels and got a result twice as large as the experimental value. The two Dutchmen had not even attempted to do that calculation but [now tried]. Einstein himself, who happened to be in Leiden, set them on the right track. The spinning electron is orbiting the atomic core which carries electric charge. In a reference system in which the electron is at rest that charge orbits around the electron. A moving charge means an electric current which generates a magnetic field. The interaction between that field and the spin magnetic moment leads to the splitting of energy levels" (ibid). Hilleth Thomas also made relativistic adjustments to the electron spin calculations regarding the factor of 2 and Pauli finally accepted the concept of intrinsic spin. In December 1925, Bohr among many others urged Uhlenbeck and Goudsmit to write a more detailed account of their finds; the paper offered here is that account. CONDITION: Original wraps. No library markings. Slight toning at inner margin of front wrap. Clean & bright inside; no marks anywhere. Very good +.…