Highly accurate measurements within the international system of units

Physics today presents a consistent model of natural phenomena based upon a coherent system of units of physical quantities. Knowledge of the most accurate values of the most important fundamental constants is essential for the traceability of units and for the refinement of theory. Of the four base units in the International System of units relevant to electrodynamics, the ampere can as yet only be realized with poor relative uncertainty of the order of 10 -6 ; the same holds true for the derived electric and magnetic units with the exception of the ohm. The accurate measurement of the gyromagnetic ratio of the proton γ p promises to yield a more accurate realization of the ampere than that achieved using the traditional current balance.