The differential cross section for Compton scattering of a circularly polarized photon by an electron with given initial spin orientation can be written as a sum of the common Klein-Nishina formula for no polarization and a term sensitive to polarization. The total cross section is $\sigma ={\sigma }_0\pm {\sigma }_1$. A measurement of the transmission of 2.62-Mev gamma rays through iron magnetized along the transmission direction relative to that through unmagnetized iron gives the absolute value of ${\sigma }_1$ for this energy, if the number ${\nu }_s$ of polarized electrons per iron atom at saturation is known. For ${\nu }_s=2.06\mathrm{}$, $\frac{{\sigma }_1}{\pi {r_0}^2}=0.089\mathrm{}\pm 0.007$. This agrees with the theoretical value 0.093. Alternatively, the theoretical ${\sigma }_1$ and the measurements would yield ${\nu }_s=1.97\mathrm{}\pm 0.15$.

The application of the method of this experiment to measurement of gyromagnetic ratios for ferromagnets is suggested, as is its application to the analysis of circularly polarized radiation.

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