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Scientists vote on first change to the kilogramme in a century

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By Claire Heffron  with Reuters
Scientists vote on first change to the kilogramme in a century

The kilogramme is one of the most important and widely used units of measurement in the world, a solid presence in kitchens and bathrooms everywhere.

And yet the very definition of the kilo is about to change radically.

On Friday 16 November 2018, measurement scientists from more than 60 countries will congregate to witness the vote on the redefinition of the International System of Units (SI), changing the world's definition of the kilogramme and other measurements.

For more than a century, the kilogramme has been defined by the weight of a cylinder of platinum-iridium at the international bureau of weights and measures in Sèvres, France.

The proposed change is to define the kilogramme by a tiny but significant value in terms of Planck’s constant, a number that describes the size of bundles of energy, rather than on one piece of metal.

The current mass of the cylinder—called Le Grand K—is constant, but its weight is not, it's shrinking slowly. It grows with the accumulation of pollution from the air and shrinks ever so slightly with each cleaning.

The official definition of a kilogramme is currently determined as the mass of a metal cylinder called the International Prototype of the KilogrammeFile

The vote will take place at the General Conference on Weights and Measures (CGPM) at the Palais des Congrès in Versailles, which is organised by BIPM.

If agreed, these changes will come into practice on 20 May 2019.

The new definition involves an apparatus called the Kibble balance, which makes use of the constant to measure the mass of an object using a precisely measured electromagnetic force.

However, while the extra accuracy will be a benefit to scientists, for the average consumer buying flour or the weight conscious, there will be absolutely no change.