Kilogram, Amp and Kelvin Redefined in Largest Units Overhaul Since 1875


The golf ball-sized metal cylinder at the heart of the world's system for measuring mass is heading into retirement. The decision was made at the final day of the 26th meeting of the General Conference on Weights and Measures (CGPM) in Versailles, France.

Describing what impact the new kilogram would have, the BIPM said: "In the same way that if you replaced the decaying foundations of a house with robust new ones, it may not be possible to identify the difference from the surface, but some substantial changes would have taken place to ensure the longevity of the property". A replica of this platinum-iridium standard was exhibited during the morning sessions of the CGPM.

For nearly 130 years, the fundamental unit of weight has been set by a cylindrical chunk of metal housed under a glass dome at the International Bureau of Weights and Measures (BIPM) near Paris, France. As part of its responsibilities, the BIPM has guarded the kilogram prototype, as well as the meter prototype, allowing other metrological institutions from around the world to come and check their standards. Along with redefining a kilogram, other base SI units were also redefined. He asked what would happen if a fingerprint got on the kilogram prototype and increased its mass.

The whole business of basing units on relics from the 19th century seems quaint-incongruent with our current world of lasers, quantum computers and space travel. "This will pave the way for far more accurate measurements and lays a more stable foundation for science".

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"If we think about what happened a few years ago, when the meter was redefined in relation to the speed of light, a few years later we saw the creation of the Global Positioning System", he said.

"This is really a pivot point for humanity", noted Jon Pratt, who has been a longtime researcher and supervisor for the NIST group working to redefine the kilogram with nature's fundamental laws. There are no more error bars here.

Scientists had to create very precise devices to measure Planck's constant, which is why the new definition took so long to publish.

Simply put, the definition of the kilogram linked to the amount of electromagnetic energy required to balance the object of the relevant mass. By 2018, the gold standard for the kilogram had lost 50 micrograms and become 99.999995 percent of its original mass. L-to-R: Eric Lin, director, Material Measurement Lab, NIST; Claire Saundry, director of International and Academic Affairs Office, NIST; Willie May, U.S. member of the International Committee on Weights and Measures and former NIST director; NIST Director and Undersecretary of Commerce Walter Copan; Barbara Cordero, finance analyst, Office of Management Policy and Resources, IO, Department of State and James Olthoff, Acting Associate Director of Laboratory Programs, NIST. Bathroom scales won't suddenly get kinder and kilos and grams won't change in supermarkets.