t’s official. The wait is finally over!
In a historic judgment, more than 60 nations came together to vote unanimously for a new system that redefines the kilogram along with three other units, the ampere (electrical current), the kelvin (thermodynamic temperature) and the mole (amount of a substance).
Since 1884, the #kilogram has been defined by an artefact – from 2019, it will be defined by fundamental constants of nature:#SIRedefinition @beisgovuk @BipmMetrology https://t.co/Z3Qp51IiIP pic.twitter.com/bBk0jpoviJ
— NPL (@NPL) November 13, 2018
On November 16, 2018, at the General Conference on Weights and Measures, in Versailles, Paris, the definition of kilogram was changed forever.
How was it defined before?
The kilogram has been defined since 1889 by a shiny piece of platinum-iridium kept in a special glass case, the International Prototype of the Kilogram (IPK), also known as Le Grand K.
It is housed at the headquarters of the International Bureau of Weights and Measures (BIPM), just outside Paris.
All modern mass measurements are traceable back to it – from micrograms of pharmaceutical medicines to kilos of apples and pears and tonnes of steel or cement.
Why is the kilogram redefined?
The problem is, the IPK 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.
That can have profound implications. If the prototype were to lose mass, atoms would, in theory, weigh more since the base kilogram must by definition always weigh a kilogram.
“We live in a modern world. There are pollutants in the atmosphere that can stick to the mass,” said Ian Robinson, a specialist in the engineering, materials and electrical science department at Britain’s National Physical Laboratory.
“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.”
What’s needed is something more constant.
— BBC News Graphics (@BBCNewsGraphics) November 16, 2018
How will it be defined now?
A kilogram will be defined by a tiny but immutable fundamental value called the ‘Planck constant.’
The new definition involves an exquisitely accurate weighing machine called the Kibble balance, which makes use of the constant to measure the mass of an object using a precisely measured electromagnetic force.
“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,” said Robinson.
While the extra accuracy will be a boon to scientists, for the average consumer buying flour or bananas, there will be absolutely no change whatsoever.
It is arguably the most significant redefinition of an SI unit since the ‘second’ was recalculated in 1967, a decision that helped ease communication across the world via technologies like GPS and the internet.
On Nov 16, the world invites a new #kilogram to town, assigning an exact fixed value to the Planck constant and bidding adieu to the precious metal artefact that has defined the unit of mass within the International System since 1889 https://t.co/0DR9nFeIWu #SIredefinition pic.twitter.com/hOUQLIgCcn
— NRC Canada (@NRC_CNRC) November 14, 2018