A Measurement of the Gyromagnetic Ratio of the Proton in a Strong Magnetic Field

We describe a measurement, accurate to a part in a million, of the gyromagnetic ratio of the proton in water, γ'_p, by the strong field method. The magnetic induction of a magnet is measured by weighing the force exerted on a rectangular current-carrying coil which has its lower end in the pole-gap. Simultaneously the precession frequency of protons in a water sample in this field is obtained by a conventional nuclear magnetic resonance technique. γ'_p is then the ratio of the precession frequency to the magnetic flux density.

The significance of the measurement lies not only in the use of the result to calibrate magnetic fields in SI units for other accurate measurements of atomic constants, but also in the realization of the SI definition of the ampere which follows from combining the result with the measurement of γ́_p by the weak-field technique.

Our result is γ'_p = 2.6751701(27) × 10⁸ s^-1 T_B169^-1 for a spherical water sample at 20°C, where the standard error in the last two digits is in parenthesis.