A further complication is that the measurement of γ′p involves the measurement of an electric current: this is invariably measured in conventional amperes rather than in SI amperes, so a conversion factor is required. The symbol Γ′p-90 is used for the measured gyromagnetic ratio using conventional electrical units. In addition, there are two methods of measuring the value, a "low-field" method and a "high-field" method, and the conversion factors are different in the two cases. Only the high-field value Γ′p-90(hi) is of interest in determining the Planck constant.
How many methods of measuring the gyromagnetic ratio are there? How do conversion factors compare in "low-field" and "high-field" methods of measuring the gyromagnetic ratio? Which method is of interest in determining the Planck constant? Why is the measurement of electric current an issue in the estimate of the gyromagnetic ratio?