FIFTY YEARS OF SPIN: Personal reminiscences

In a one-page Letter to the Editor of Naturwissenschaften dated 17 October 1925, Samuel A. Goudsmit and I proposed the idea that each electron rotates with an angular momentum ℏ/2 and carries, besides its charge e, a magnetic moment equal to one Bohr magneton, eℏ/2mc. (Here, as usual, ℏ is the modified Planck constant, m the mass of the electron and c the speed of light.) Sam, in his accompanying article, tells something of those times, fifty years ago. We have often talked about the circumstances that led to our idea, but it was mainly Goudsmit's recollections that have appeared in print before now—they are, however, not readily accessible in English. Although I gave a short account of the discovery of the spin as a part of my inaugural address for the Lorentz professorship in Leiden in 1955, it therefore appears to be my turn to reminisce.