Sorry, Primary Kilogram, But We Are Dumping You

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January 3, 2019 | Originally published by Date Line: January 3 on

The physical kilogram, the last of the physical primary-standard artifacts, has been made obsolete. The world’s metrology and standards organization voted to define all physical standards in terms of just seven universal constants.

In November 2018, in Versailles, France, representatives from the National Institute of Standards and Technologies (NIST) and 60 countries unanimously voted to officially transform the international system of units that underpins global science and commerce. The decades-long quest for a measurement system based entirely on unchanging fundamental properties of nature was officially confirmed for the defining metrology framework used throughout the world. This is the culmination of a larger plan, many years in the making, to define all needed standard values in terms of seven physical constants.

This SI or International System of Units (abbreviated from the French Système international (d”unités)) has seven “base” units such as the second, meter, and kilogram, from which all other measurement units, including the watt and volt, can be derived. These base units define the legal and scientific ultimate meaning of time, distance, mass, electric current, temperature, the amount of a substance, and luminous intensity.

Engineers and scientists know the importance of metrology and primary standards for basic units of time, distance, mass, and more. Decades ago, the second was defined in terms of the Earth’s orbital period (what…it changes a little year-to-year?), the highest-level meter standard was a platinum-iridium bar in Paris with tiny scratch marks (it’s very hard to gauge those, that’s for sure!), and the primary kilogram—called the International Prototype Kilogram (IPK)—was a platinum-iridium cylinder kept in vacuum jar (also in Paris) and only used every few decades to calibrate many secondary kilogram masters (Fig. 1). Complicating its credibility was the verified assessment that this highest-level IPK was apparently losing a few but very precious nanograms every few years, for various unclear reasons.

But the kilogram evaded being “retired” because there was no way to make an independently reproducible equivalent, and it remained the lone remaining artifact, until recently. Now, decades of worldwide leading-edge R&D bolstered by experimentation and instrumentation finally reached a point when the “master” kilogram has been officially retired and replaced with a reproducible equivalent, as part of a measurement system based entirely on unchanging fundamental properties of nature.

The Kibble Ballance