Part I--from the Current "Kilogram Problem" to a Proposed Definition (The Kilogram in the "New SI"‪)‬

The metric system of measurements has served the international scientific and technical communities well since its inception over 200 years ago. By the mid 1800s, three so-called "base units" were in place, for measuring distance, mass, and time (centimetre, gram, second, or CGS system). These base units evolved into the metre, kilogram, and second (MKS system). The ampere became the fourth base unit in 1946. In 1954, the kelvin and candela became new base units. Finally, in 1971, the mole became the seventh base unit, for amount of substance. The definitions of each base unit have undergone continuous evolution, corresponding to improvements in measuring capabilities, and recognition of shortcomings of prior definitions. As stated by BIPM, "The SI is not static but evolves to match the world's increasingly demanding requirements for measurement." Of the seven base units in existence today, only one of them is based on a physical artifact. The current definition for the kilogram is embodied in Le Grand K, the platinum-iridium cylinder maintained by the BIPM in Sevres, near Paris. Comparisons with virtually identical artifacts suggest that the mass of Le Grand K may have changed by 50 micrograms, or possibly even more, from its date of creation in 1884. Starting with the 1963 redefinition of the second and the 1983 redefinition of the metre, there has been a call to replace the definition of the kilogram with a more suitable one, using some "invariant of nature" to replace the artifact kilogram. (1) Since the perceived weakness of the current SI definitions of other units such as the ampere, mole, and candela "derives in large part from their dependence on the kilogram ... the definition of the kilogram is thus central to the more general problem of improving the SI." (2) This so-called "kilogram problem" is the subject of this paper.