A weighty issue: scientists redefine the kilogram after 129 years

Sarah Knapton
A replica of the International Prototype Kilogram is pictured at the International Bureau of Weights and Measures (BIPM) - REUTERS

The kilogram will no longer be measured against an actual weight, after scientists voted to start using an electromagnetic current.

Since 1889, a kilogram has been defined by a single lump of platinum-iridium which is housed inside three glass bell jars at the headquarters of the International Bureau of Weights and Measures (BIPM) , just outside Paris.

But the master copy, known as ‘Le Grand K’, has been picking up microparticles of dust, or losing mass in cleaning, causing consternation for scientists using it to measure ever more accurate weights.

Now, after a week-long meeting at the Palace of Versailles representatives of 60 nations agreed to redefine the kilogram based on the unchanging value of the ‘Planck constant.’

Instead of checking it against an actual weight, scientists can now find an exact kilogram by measuring the amount of electricity needed to lift it, using a special set of scales known as The Kibble balance.  

Scientists have been trying for decades to define a constant value for the kilogram that is derived from laws of physics, in the same way they have done for other standard units.

For example, a metre is not defined as 100 centimetres but "the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second".

Scientists Yuning Duan and Gert Rietveld celebrate after the vote on the redefinition of four base units of the International System of Units Credit: Reuters 

Describing what impact the new kilogram would have, the BIPM said: "In the same way that if you replaced the decaying foundations of a house with robust new ones, it may not be possible to identify the difference from the surface, but some substantial changes would have taken place to ensure the longevity of the property."

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

Martin Milton, director of the BIPM, said: “The SI redefinition is a landmark moment in scientific progress.

"Using the fundamental constants we observe in nature as a foundation for important concepts such as mass and time means that we have a stable foundation from which to advance our scientific understanding, develop new technologies and address some of society's greatest challenges."

Barry Inglis, who heads the committee for weights and measures, said the implications were immense.

"We will now no longer be bound by the limitations of objects in our measurement of the world, but have universally accessible units that can pave the way to even greater accuracy, and even accelerate scientific advancement," he said.

The new definitions agreed by the BIPM will come into force on May 20, 2019.