Kilogram, ampere, kelvin and mole redefined from May 2019

[gary]

Originally Posted by Julianne Pepitone, IEEE Spectrum, 16 Nov 2018

Our standards of measurement seem, well, standard. A meter is a meter. A second is a second. The units by which we measure our world simply are—they feel as immutable as an ocean wave.

But these longtime standards are set to change. A governing body unanimously voted Friday for the biggest ever overhaul of the international system of units (SI), which will redefine four base units: the ampere, kelvin, kilogram, and mole.

“It’s a moment of quite major significance in our obsessive little world of metrology,” says Jon Pratt, chief of the National Institute of Standards and Technology (NIST)’s Quantum Measurement Division of the Physical Measurement Laboratory, who managed a team that has worked for eight years on equipment and projects related to the SI unit overhaul.

“We’re achieving the completion of an arc that started when the French were first thinking up the metric system,” Pratt adds. “They were striving for a universal measurement system. And now we are basing units on the fabric of the universe.”

Originally Posted by Julianne Pepitone, IEEE Spectrum, 16 Nov 2018

Perhaps the biggest—and certainly the buzziest—change is to the kilogram, the last base unit to be defined by a physical artifact. The International Prototype Kilogram, or Le Grand K as it is affectionately known, is a cylinder made of platinum and iridium, housed in a trio of bell jars in Paris and held under lock and key. It’s taken out only to measure against official replicas of the kilogram, in an intensely careful process.

In a keynote speech Friday ahead of the vote, Nobel laureate Bill Phillips of NIST’s Gaithersburg, Md., group highlighted the limitations of this setup as he held up a kilogram replica to the assembled crowd.

“If this were the real kilogram that I were holding in my hands, the fingerprints I put on this kilogram would increase the mass,” Phillips said. “But of course it can’t increase the mass, because by definition this is a kilogram. So you would all lose weight!”

With the revamp, the kilogram will now be defined in terms of the Planck constant—which itself will be given a fixed numerical value—and can then be realized by methods like the Kibble (watt) balance or the Avogadro constant.

Article here :-
https://spectrum.ieee.org/tech-talk/...mpere-and-more