Facts at your Fingertips: Redefining the Kilogram Standard
By Scott Jenkins |
The kilogram is the last of the SI units (International System of Units) still defined by a physical object, rather than defined in terms of universal fundamental constants of nature. However, this will change in November 2018, as a multi-year effort culminates in the adoption of a new definition for the kilogram that is based on Planck’s constant. This one-page reference describes the effort to redefine the kilogram standard, shifting the definition from a physical object, which can change over time, and to a definition according to stable and reproducible constants of nature.
The kilogram is currently defined as the mass equal to a polished cylinder of platinum and iridium known as the International Prototype of the Kilogram (IPK; photo). Cast in 1879, it is currently housed at the Bureau International des Poids et Mesures (BIPM; Sèvres, France; www.bipm.org). According to the U.S. National Institute of Standards and Technology (NIST; Gaithersburg, Md.; www.nist.gov), “The accuracy of every measurement of mass or weight worldwide depends on how closely the reference masses used in those measurements can be linked to the mass of the IPK” .
Re-defining the kilogram
With improved measurement technologies,…