Maždaug KiloGramas

Anjana Ahuja

 YESTERDAY

Subject to an international vote, the definition of the kilogramme will change. The standard unit of mass will no longer be the International Prototype Kilogram, a 19th-century ingot of platinum-iridium encased in three bell jars in an underground vault near Paris. Instead, the kilo will be calculated using Planck’s constant*, an unbelievably tiny number that plays an outsized role in quantum physics.

It is a historic moment: the kilo is the last remaining major unit to be based on a physical object. Friday’s vote at the International Bureau of Weights and Measures in France will see the measurement, which seems so solid, benchmarked against a fundamental but intangible constant of nature.

The International Prototype Kilogram, a 19th-century ingot of platinum-iridium, is not accurate enough for modern metrologists 

The writer is a science commentator

....

No drinker likes to be short-changed, as the Magna Carta recognised more than 800 years ago: “Let there be one measure for wine through our whole realm, and one measure for ale, and one measure for corn, to wit ‘the London quarter’; and one width of cloth, whether dyed, russet or halberget . . . ” 

*  As of 2015, the best measured value of the Planck constant was:^[2]

${\displaystyle h=6.626\ 070\ 040(81)\times 10^{-34}\ {\text{J}}{\cdot }{\text{s}}=4.135\ 667\ 662(25)\times 10^{-15}\ {\text{eV}}{\cdot }{\text{s}}}$.

In November 2018 BIPM** decided to redefine kilogram by fixing the value of Planck constant. Starting 20th of May 2019, the new value is exact, ${\displaystyle h=6.626\ 070\ 15\times 10^{-34}\ {\text{J}}{\cdot }{\text{s}}}$

** The International Bureau of Weights and Measures (French: Bureau international des poids et mesures (BIPM)) is an intergovernmental organization that was established by the Metre Convention, through which member states act together on matters related to measurement science and measurement standards. The organisation is usually referred to by its French initialism, BIPM. The BIPM's headquarters is based at Sèvres, France. It has custody of the International Prototype of the Kilogram and houses the secretariat for this organization as well as hosting its formal meetings.