Kilogram gets new definition as ′Le Grand K′ tipped off the scale | News | DW | 16.11.2018
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Kilogram gets new definition as 'Le Grand K' tipped off the scale

Scientists have agreed to define a kilogram in terms of an electric current instead of "Le Grand K," a metal cylinder kept in a Paris vault. But the new definition will have no noticeable impact on most people's lives.

Scientists on Friday agreed to update the definition of a kilogram by making an "electronic kilogram" the new baseline measure of mass.

The kilogram has been defined since 1889 by the weight of "Le Grand K," a cylinder of polished platinum-iridium alloy kept in a Paris vault.

Scientists have now agreed to replace the metal kilo by a definition based on Planck's constant, named after the German physicist Max Planck and one of the most celebrated equations in physics.

The change will not have any noticeable impact on people's weight or for that matter on the weight of grocery items in supermarkets.

Read more: Our vanishing world of physical units

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.

"In the present system, you have to relate small masses to large masses by subdivision. That's very difficult — and the uncertainties build up very, very quickly," said Ian Robinson, a specialist in the engineering, materials and electrical science department at Britain's National Physical Laboratory.

"One of the things this (new) technique allows us to do is to actually measure mass directly at whatever scale we like, and that's a big step forward."

Why did scientists redefine kilogram?

Scientists found that "Le Grand K" did not always weigh the same and felt the need for a more accurate and consistent definition.

Even inside its three glass bell jars, the metal kilo was susceptible to dust particles and other atmospheric pollutants.

"We live in a modern world. There are pollutants in the atmosphere that can stick to the mass," Robinson said.

"So when you just get it out of the vault, it's slightly dirty. But the whole process of cleaning or handling or using the mass can change its mass. So it's not the best way, perhaps, of defining mass."

The new formula solves this problem and is expected to be more accurate when measuring very, very small or very, very large masses.

The updated definition will spare countries the need to occasionally send their kilos back to France for calibration against "Le Grand K." In the United States, a kilo will still weigh 2.20462 pounds.

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What will happen to 'Le Grand K'?

The new definition will render "Le Grand K" and its six official copies redundant.

Even in retirement, the metal kilos — collectively known as "the heir and the spares" — will be kept in the high-security vault on the outskirts of Paris.

This will allow scientists to keep studying them, to see whether their masses gradually change over time.

The new definition is set to come into effect on May 20, 2019.

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ap/sms (AP, Reuters, dpa)

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