We describe a 3 He magnetometer capable to measure high magnetic fields (B > 0.1 Tesla) with a relative accuracy of better than 10 -12 . Our approach is based on the measurement of the free induction decay of gaseous, nuclear spin polarized 3 He following a resonant radio frequency pulse excitation. The measurement sensitivity can be attributed to the long coherent spin precession time * 2 T being of order minutes which is achieved for spherical sample cells in the regime of "motional narrowing" where the disturbing influence of field inhomogeneities is strongly suppressed. The 3 He gas is spin polarized in-situ using a new, non-standard variant of the metastability exchange optical pumping. We show that miniaturization helps to increase * 2 T further and that the measurement sensitivity is not significantly affected by temporal field fluctuations of order 10 -4 .

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