The world has just redefined the kilogram

Kilogram Amp and Kelvin Redefined in Largest Units Overhaul Since 1875


Le Grand K, the prototype defining a kilogram of mass across the world, is retiring as scientists have just changed the definition of kilogram.

The new method, decided at the General Conference on Weights and Measures in Paris on Friday, will be a formula called the "Planck constant".

They also voted to update definitions for the ampere (electrical current), the kelvin (thermodynamic temperature), and the mole (amount of a substance).

Guardians of weights and measurements may be about to kill off the kilogram, as it is now defined. The revised SI will maintain its relevance by facilitating technical innovations.

After binding to the Planck constant reference pounds will not be as much weighing - the weight of the cylinder will differ from a kilogram of ±10 micrograms. "I hope that such votes will be possible for many other issues in the world", said Sebastien Candel, president of the French Academy of Sciences.

And today, the worldwide General Conference on Weights and Measures is meeting in Versailles to vote on whether the kilogram will be redefined.

Since 1889, a kilogram has been defined by a shiny lump of platinum-iridium kept in a special glass case and known as the worldwide prototype of the kilogram.

Quantum phenomena that are identical everywhere are already used to define the second, which is the SI unit for time, and the meter, the SI unit for distance.

The metre is no longer defined by a rod of metal, but by the distance light travels in a set, and very small, fraction of a second. The Planck constant is ready for use everywhere and always. The machine measures the weight of an object by the strength of the electric current and voltage required to produce a compensating force, equalising one force with another.

But the new formula-based definition for the kilogram will have multiple advantages over the precision-crafted metal lump that set the standard from the 19th century to the 21st, through periods of stunning human achievement and stunning follies, including two world wars.

The definition of the kilogram for more than 130 years, the International Prototype of the Kilogram (IPK), a cylinder of a platinum alloy stored at the BIPM in France, will now be retired. It is also created to facilitate today's constant technical innovations.

In a similar way, the SI unit for the ampere will now be based on the constant for the charge of the electron.

Things are however going to change for the metrologists, who specialise in the science of measurement. "As an historic artefact that has been under study for 140 years, it will continue to be important for researchers for years to come", it says.

Pharmaceutical companies are now measuring mass at microgram levels, which is only a billionth of a kilogram.

For further information visit NIST's SI Redefinition website.

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