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The History of the Discovery and Acceptance
of the Notion of the Spin of an Electron

In the 1920's physicists searched for quantitative explanations of the fine structure of the spectra of elements. Empirical formula had been developed that gave amazingly accurate computation of the empirical measurement of the spectrum of hydrogen. Bohr's theory of the hydrogen atom gave a solid justification for those empirical formulas. However when an electric field or magnetic field is introduced into the measurement apparatus the lines which were single become double or triple. There are shifts in the wavelengths of spectral lines associated with outside influences. These shifts and separations in wavelength of the double or triple lines became the target for the physics theorists of the time. But first it is important to note the result of a crucial experiment.

The Stern-Gerlach (S-G) Experiment

In 1922, years before the development of new quantum physics, the German physicists Otto Stern and Walther Gerlach published the results of one the most crucial experiments in physics. They passed a beam of silver atoms through a magnetic field with a gradient in intensity perpendicular to the direction of the beam. Classical physics indicates that the beam would be spread out in a continuous normal distribution about a mean of zero deviation from the direction of the incident beam. Instead what was found was the beam was split into two in the direction of the gradient of the magnetic field. This was an unexpected result according to classical physics. The Russian physicist Lev Landau said of the result that there is nothing in classical physics which would account for such a phenomenon. What Stern and Gerlach were looking for was a quantization of the direction of the angular momentum vectors of atoms. The results of their experiment appeared to give some confirmation their conjecture, but subsequent developments led to the explanation of the results in terms of the spin of the valence electron(s) of the atoms.

(To be continued.)

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