Mass Metrology and the International System of Units (SI)

The International System of Units (SI) is widely used in science, industry, and commerce because it caters simultaneously to the needs of all. In the early twenty-first century, this means defining the units of time, length, mass, and electricity in terms of the fundamental constants of physics, and then "realizing" these definitions to sufficient accuracy on the human scale of the second, meter, kilogram, and ampere. This program has already been successful except for the kilogram, which is still defined in terms of an artifact constructed in the late nineteenth century. Although quantum-based electrical standards are widely used, the SI voltages or resistances produced by these standards depend on the values of constants that are at present based on experimental values derived from the artifact kilogram. This chapter presents the current state of affairs, which is unsatisfactory, and proceeds to describe work that will lead to a redefinition of the kilogram, probably in terms of a fixed value for the Planck constant.