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November 17, 2018

Definition of Kilogram changed after 130 years

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At the 26th General Conference on Weights and Measures held at Versailles, Paris, the 130 year old definition of Kilogram was changed forever. Since 1889, the Kilogram has been defined by the weight of a Platinum-based ingot called "Le Grand K". It is kept in a special glass case at the headquarters of the International Bureau of Weights and Measures (BIPM), near Paris.
But today, more than 60 nations voted unanimously to get rid of it in favour of defining a kilogram in terms of an electric current. They also voted to redefine the ampere (electrical current), the kelvin (thermodynamic temperature) and the mole (amount of a substance).

What was the problem with the present definition
The 'Le Grand K' doesn't always weigh the same. Even inside its three glass bell jars, it picks up microparticles of dirt and is affected by the atmosphere. Sometimes it needs cleaning, which can affect its mass.

What is the new definition of Kilogram ?
From now onwards, a Kilogram will be defined by a tiny but immutable fundamental value called the 'Planck constant.' The new system uses a Kibble balance (an exquisitely accurate weighing machine) where weight is balanced against an electromagnetic force. Conceptualised in 1975 by late British physicist Bryan Kibble, the latest Kibble balance can measure Planck constant with uncertainties in a few parts per billion.

The changes will come into effect from World Metrology Day (20th May 2019).

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