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 that all other mass values in the world are tested against. 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.
Last Measurement Artifact
One of the most important things to know about the International Prototype Kilogram or IPK is that it is the only unit of measure that is still defined by an artifact. All other units of measure, such as length, can be defined by a mathematical equation. Being able to define a measurement by an invariant quantities of nature allows others to be able to reproduce the measurement vs testing against an artifact.
Why Change the Artifact
Over the past century or so, world-wide 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.
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 determine of Planck’s constant.
Making the Change
The CIPM is due to meet in November 2018 to review the progress in testing the accuracy of Planck’s constant as a replacement to the IPK. Based on current progress, it is anticipated that the change will be adopted in May 20, 2019.
There are two good articles that include in-depth information on this upcoming change:
General information on the upcoming change. This article is based on an interview with NIST at Public Radio International. The audio of the interview is included with the article.
The below link is NIST’s announcement regarding its progress testing Planck’s constant. It also includes a more in-depth explanation of the constant.
- Wikipedia, https://en.wikipedia.org/wiki/Kilogram
- NIST article – New Measurement Will Help Redefine International Unit of Mass – https://www.nist.gov/news-events/news/2017/06/new-measurement-will-help-redefine-international-unit-mass