Landmark change to the definition of kilogram approved

Source Enrico Massa and Carlo Sasso 

		News		Why the kilogram is changing forever By Katrina Krämer16 November 2018

In this photo taken on Wednesday, October 17, 2018, a replica of the International Prototype Kilogram is pictured at the International Bureau of Weights and Measures, in Sevres, near Paris. Things are all set to change now.

A historic decision was taken this week at the General Conference on Weights and Measures in Versailles, France, organized by the International Bureau of Weights and Measures (BIPM), which marks the biggest overhaul of the International System of Units (SI) since 1875.

And kilogram is not alone.

HUMANKIND IS ABOUT to sever one of the links between its present and its past.

Humanity uses seven main measurements units: the metre for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for the amount of a substance and the candela for luminous intensity.

Explained: How much is a kilogram?

But why change the kilogram at all? "This will pave the way for far more accurate measurements and lays a more stable foundation for science". But now, it will be retired and replaced by the Planck constant - the fundamental constant of quantum physics.

"...all the definitions will be based on what we call the fundamental constants of nature", metrologist Estefanía de Mirandés of the BIPM tells

What happens to Le Grand K?

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Le Grand K was compared with the various copies only once every 40 years, which made the calibration potentially inaccurate.

But physical objects can lose atoms (actually steaming) or, alternatively, to absorb molecules from the surrounding air, so over the past century, the mass standard was changed to a few tens of micrograms.

"We put in place a system that doesn't depend on something that is 140 years old", Milton said. Sometimes it needs cleaning, which can affect its mass. Many in the room were especially anxious to say goodbye to the global prototype of the kilogram, which was created in 1889.

The ideal kilogram, as also the copies, however showed slight changes over time. It has been the world's one true kilo, against which all others were measured, since 1889.

In fact the changes were so small they were about 50 parts out of a billion, or the weight of an eyelash. Though the change was negligible, Dr Stuart says this was "not acceptable from a scientific point of view". "So even though Le Grand K is fit for goal at the moment, it won't be in 100 years' time". Yet, despite its heavy security, the cylinder has begun to wear away, causing slight inaccuracies in the way we define and measure a kilogram.

Instead of checking it against an actual weight, scientists can now find an exact kilogram by measuring the amount of electricity needed to lift it, using a special set of scales known as The Kibble balance.

So the scientists gathered Friday at the General Conference on Weights and Measures chose to retire Le Grand K and move on to something that couldn't be changed: the mathematical concept known as Planck's constant.

It was originally called a "watt" balance, the SI unit of power. This mass was then represented by a cylinder - the prototype that was adopted as a unit of weight by other countries.

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