Superconductivity and the SI (Metric) Systems Based on Fundamental Constants
Edwin R. Williams
Superconducting measurements that help connect the unit of mass, the kilogram, to the Planck constant h are bringing the time when the scale for all units in the International System of Units (SI) can be defined directly through fundamental and atomic constants. The SI, sometimes called the metric system, was adopted internationally at the signing of the Treaty of the Meter in 1875 and has served science, engineering and commerce very well ever since. One reason for its success is that the system is adaptive to change. New science and technologies, such as those associated with superconductivity, have provided scientists with a connection between the atomic world that is governed by quantum mechanics and the more familiar world of macroscopic measurements of voltage and current. We need to extend this link to the kilogram which is defined as the macroscopic mass of a metal cylinder made of platinum-iridium that, since 1889, has been kept in a vault at the Bureau International de Poids et Mesures, BIPM, in S`evres, a suburb of Paris. In 1967 developments in atomic timekeeping allowed the General Conference for Weights and Measures (CGPM), an inter-governmental organization which oversees the SI, to set the scale for the second by fixing the value of the ground state hyperfine transition frequency of the cesium 133 atom.
100 Years of Superconductivity
Chapman & Hall/CRC Press of the Taylor & Francis Group, Danvers, ME