From Artifact to Constant – Redefining the Kilogram

In the Saint-Cloud, France facility of the International Bureau of Weights and Measures sits a small platinum-iridium cylinder.  It is stored under two glass bell jars and, is kept under lock and key.  This piece of metal is the International Prototype Kilogram.  Since 1889, it has been the standard against which all other mass values in the world are tested.  Multiple copies of this prototype have been made and are distributed throughout the world.  The United States has five copies: K4, K20, K79, K85 and K95.  Copies K4 and K20 are stored at the US National Institute of Standards and Technology (NIST) in Gaithersburg, MD. All traceable weights in the US can be traced back to these standards.

Over the past century or so, worldwide national standards have been tested against the IPK.  The test results have shown some instability in the measurement between the IPK and the copies.  The stability of the IPK is crucial because the kilogram unpins much of the SI system of measurement as it is currently defined and structured¹. This instability has prompted the International Committee for Weights and Measures (CIPM) to “take note of an intention” that the kilogram be defined in terms of the Planck constant.

Plank’s Constant

Testing of Planck’s constant, a value from quantum mechanics expressed as h = 6.626069934 x 10⁻³⁴ kg∙m²/s, is ongoing.  Planck’s constant enables researchers to relate mass to electromagnetic energy². The researchers at NIST announced in June 2017 that they have made its most precise determination yet of Planck’s constant. These results, along with those of other research facilities, will define the most precise determination of Planck’s constant.