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, metrologists have shown that the masses of IPK and other reference artifacts have diverged by about 50µg over the past 100 years. Even if stored carefully and used sparingly, physical objects can lose or gain atoms over time. Defining the kilogram based on natural constants would, in theory, allow the exact kilogram measure to be available globally and in a way that is stable over time.
Scientists have known for some time that the kilogram could be defined using Planck’s constant ( h), but techniques for measuring the constant were not able to generate the precision needed to allow a new definition to replace the physical kilogram standard.
Named for German physicist Max Planck, h is a proportionality constant between the minimum increment of energy for a photon, and the frequency of its associated electromagnetic wave. Planck’s constant, a central component of quantum mechanics, allows researchers to relate mass to electromagnetic energy through mass-energy equivalence (E = mc2).
The history the kilogram standard can be traced to 1791, when, after a decree from King Louis XVI of France regarding the definition of length, the kilogram was defined as the weight of one cubic decimeter (based on the new length standard) of distilled water at its melting point. By 1799, French scientists of the time had fabricated a cylinder of platinum equal to that mass, and it became known as the kilogram of the archives (KA). A more detailed history of the kilogram can be found in Ref. 2.
In 1875, BIPM was established, as international teams of metrologists tried to come up with a successor to the KA. By 1880, metrologists had constructed a 90%-platinum, 10%-iridium mass that equaled the mass of KA according to the best measurement techniques of the time. It would become known as the IPK. Housed at the BIPM along with six official copies, the IPK has served as the definition of the mass of the kilogram since it was sanctioned in 1899.
Measuring Planck’s constant
To measure Planck’s constant with a sufficiently low degree of uncertainty, teams of scientists have been using two separate methods. The primary method involves a Kibble balance, an instrument designed for accurate measurements of h. Previously known as Watt balances, such instruments were renamed after the death in 2016 of their inventor, British metrologist Bryan Kibble. The other method involves counting the number of atoms in a silicon sphere to determine Avogadro’s constant (NA), which allows an alternate route to determine h.
According to NIST, a Kibble balance uses electromagnetic forces generated by a coil of wire between two permanent magnets to balance a mass . The Kibble balance operates in two modes: first, electrical current is passed through the coil, which creates a magnetic field that interacts with the permanent magnets. This generates an upward force to balance the mass. In the second mode, the wire coil is lifted at constant velocity, inducing a voltage that is proportional to the magnetic field strength. With accurate measurements of current, voltage and velocity, enabled by the Kibble balance, Planck’s constant can be calculated to a high degree of certainty, because it is proportional to the amount of electromagnetic energy needed to balance the mass.
The new NIST measurement of Planck’s constant is 6.626069934 × 10−34 kg∙m2/s, with an uncertainty of only 13 parts per billion (ppb). The agreement between the h values determined by the two methods (Kibble balance and Avogadro method) is very good, according to NIST scientists, with measurements differing by only a few parts per billion.
Work necessary to adopt the new kilogram definition is now in place, and the formal adoption will be announced at the 26th General Conference on Weights and Measures meeting in November 2018 .
Groups within the BIPM have been working on the adoption of the new definition and helping to integrate the new definition into familiar quality-control systems used by industry and science. Also, there will need to be an education campaign to explain these changes to user communities.
1. Kilogram: Introduction, National Institute of Standards and Technology (NIST; Gaithersburg, Md.; www.nist.gov), Information on new kilogram standard, accessed August 2018.
2. Davis, Richard S. and others, A brief history of the unit of mass: continuity of successive definitions of the kilogram, Metrologia, 53, A12, 2016.
3. Bureau Internationale des Poids et Mesures (Sevres, France; www.bipm.org), information on redefining kilogram standard, accessed July 2018.
4. Abbott, Patrick, NIST, personal communications
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