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We have measured the ratio of the NMR spin precession frequencies of optically pumped low pressure helium-3 and of protons in water, using free induction decay. This was done by direct substitution in a flux density of 0,1 T of samples in an accurately spherical 25 mm diameter sample cell, without removing the sample cell from the magnet. The water measurements were corrected to 25°C. The two sets of measurements were each referred to a 10 mm diameter spherical reference water sample which was ratioed out. A new method was devised to measure and correct for the different averaging of the residual inhomogeneities in the magnetic field by the helions and by the protons in the water. Our final result is μh (3He)/μ'p= − 0,761 786 131 3(33). Using the CODATA 1986 recommended value for γ́p gives γh/2π = 32,434 092 1(97) MHz T-1. Combining the result with a measurement of γh (3He)/γp (H2 gives the shielding factor of H2 as σH2 = 26,350(19) × 10-6, in good agreement with a theoretical value. Combining our result with a measurement of μ́p/μB yields μh (3He)/μB= − 1,158 671 471(14) × 10-3.

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