A new precision measurement of the ratio of the proton NMR frequency to the cyclotron frequency of free electrons in the same magnetic field, Ωp/Ωe=γpmc/e, has been performed. A cyclotron resonance linewidth of 5 ppm only and a reduction of the large space charge shifts, amounting up to the order of 50 ppm in earlier published measurements, enabled a high accuracy. The final result is Ωp/Ωe=1.5209945×10-3±0.5 ppm, where the NMR is referred to a long cylindrical sample of H2O, 0.2 m CuSO4. Corrected to the free proton, we get Ωp/Ωe=1.5210329×10-3±0.6 ppm. Combination of our result with an EPR-measurement of Lambe yields a free electron g-factor of g=2(1+(α/2 π)-(0.30±0.09) (α/π)2) in good agreement with the theoretical value g=2(1+(α/2 π)-0.328(α/π)2±...). © 1968 Springer-Verlag.
