Scientists have unanimously voted to change the official definition of a kilogram.
It is the first change to the official definition in over a century.
The vote took place this morning at the 26th General Conference on Weights and Measures in Versailles in France.
Metrology experts from nearly 60 countries decided to make an ‘electronic’ kilogram the new baseline for mass measurement.
Up till now, all the world’s scales – including those that weigh in pounds and ounces – were calibrated against a small cylinder of titanium alloy which has been held in a triple locked Parisian vault since 1889.
‘Le Grand K’ has only been removed from its case four times in the last century.
While it was viewed as the one true kilogram in the world, several exact copies were made and placed in national metrology laboratories around the world.
Since 1889, the kilogram had been defined by a piece of metal held in a vault in the International Bureau of Weights and Measures laboratory in France. Because it was a physical object, it was subject to environmental effects and lost 500 micrograms in weight. #SIRedefinition pic.twitter.com/Dt3GCx0LYf
— NSAI (@NSAI_Standards) November 16, 2018
Ireland’s copy is housed in the National Metrology Laboratory in Glasnevin which is run by the National Standards Authority of Ireland (NSAI).
The problem with the system however, was that the weight of the ‘Le Grand K’ appears to have changed ever so slightly over the years.
Even inside its protective case the object gets dusty and dirty – meaning it grows heavier. Meanwhile, giving it a wash will make it lighter.
Paul Hetherington, Manager of the NSAI's National Metrology Laboratory said periodical checks of copies against the original threw up some interesting findings.
"Because it is a physical artefact, 'Le Grand K' is subject to surface contamination and environmental effects which means that its subject to drift and change in its mass value,” he said.
“That's not an issue if you're weighing yourself on the bathroom scales but it is significant for industries that require accurate precision measurement, such as medical device manufacturers, computer chip manufacturers and pharmaceutical manufacturers.”
Just as the metre - once the length of a bar of platinum-iridium, is now defined by the constant speed of light in a vacuum, the kilogram needed to be redefined in terms of something that would never change. #SIRedefinition pic.twitter.com/cQZFpFm9O9
— NSAI (@NSAI_Standards) November 16, 2018
Scientists estimate that ‘Le Grand K’ has lost around 20 billionths of a gram – approximately the weight of an eyelash – over the years.
Following today’s vote, the kilogram will now be defined in terms of a fundamental value called Planck's constant.
This is a completely unchanging value that relates the energy in one quantum of electromagnetic radiation to the frequency of that radiation.
The NIST-4 watt balance. Image: US National Institute of standards and Technology
The new definition is measured using a machine called the Kibble balance.
“This historic change will be the largest single shift in international measurement since the International Metre Convention was signed in 1875,” said Geraldine Larkin, Chief Executive of NSAI.
“This new definition will stand the test of time and remain robust for upcoming advances in science and technology."
“As technology gets smaller and smaller, more precision and accuracy are required.
“Scientists expect this change will spur technological innovation and lower the cost of many high-tech manufacturing processes.
“For computer and mobile device manufacturers, to ship building and medical devices production, this is a significant step into the future.”
The kilogram is just the latest standard unit of measurement to be updated as scientists work to redefine all base measurement units in terms of fundamental constants of nature.
The metre - once the length of a bar of platinum-iridium, also kept in Paris – was in 1983 changed from “one ten-millionth the distance from the Equator to the North Pole” to the “distance travelled by light in a complete vacuum in a tiny fraction of a second.
