Baker checking out his scales

World weighs in on new kilogramme; key measurement units redux

Image credit: Reuters

The international system of measurements has been overhauled, with new definitions for the kilogramme and other key units.

At a meeting in Versailles, France, countries voted to approve the wide-ranging changes that underpin vital human activities such as global trade and scientific innovation.

The most closely watched change was the revision to the kilo, the measurement of mass. Before today, it has been defined as the mass of a physical lump of platinum-iridium, the so-called Grand K, which is kept in a secure vault on the outskirts of Paris.

This hunk of metal has been the world’s one true kilo, against which all others were measured, since 1889. 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.

Periodically, it has been brought out of storage for analysis and recalibration - even the slightest hint of dust could affect all other weights based on its mass - but it is now finally being retired and replaced by a new definition based on a scientific formula.

The long-recognised problem with it has simpy been that the “international prototype kilogram” doesn’t always weigh the same. Despite being housed inside three glass bell jars, it still gets dusty and dirty and is affected by the atmosphere.

“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.”

The vote was greeted by sustained applause and cheers after the 50-plus countries in attendance said yes, or “oui”, when asked one by one for their decision.

Nobel prize winner William Phillips called it “the greatest revolution in measurement since the French revolution”, which ushered in the metric system of metres and kilogrammes.

The new definition involves an apparatus called the Kibble balance, which makes use of the constant to measure the mass of an object using a precisely measured electromagnetic force.

“In the present system, you have to relate small masses to large masses by subdivision. That’s very difficult - and the uncertainties build up very, very quickly,” Robinson said.

“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.”

He said it had taken years of work to fine-tune the new definition to ensure the switchover will be smooth.

The metal kilo is being replaced by an “electronic kilogram” as the new baseline measure of mass, adhering to a definition based on Planck’s constant, part of one of the most celebrated equations in physics, one sufficiently complicated to be largely beyond the comprehension of the layman. The updated definition will at least spare nations the need to send their physical metal kilos back to France for calibration against the Grand K.

Just as the metre - once accepted to be the length of a physical bar of platinum-iridium, also kept in Paris - is now defined by the constant speed of light in a vacuum, so a kilogram will now be defined by the tiny but immutable fundamental value called the “Planck constant”.

Of course, this seismic scientific shift in measurement will have no discernable impact for most people. Bathroom scales will remain brutally honest and the weight of bananas and potatoes in supermarkets will not change. That being said, just as the redefinition of the second in 1967 helped to ease communication across the world via technologies like GPS and the internet, experts say the change in the kilogram will be better for technology, retail and health.

However, today’s decision does mean redundancy for the Grand K and its six official copies. The new formula-based definition of the kilogramme has multiple precise advantages over the precision-crafted metal lump.

Unlike a physical object, the formula cannot pick up particles of dust, decay with time or be dropped and damaged. It also is expected to be more accurate when measuring very small or very large masses.

Even in retirement, the Grand K and its official copies - collectively known as “the heir and the spares” - will still be kept in the high-security vault where they are stored, because scientists want to keep studying them to see whether their masses gradually change over time.

Scientists for whom the update represented decades of work clapped, cheered and even wept as the 50-plus nations approved the update. Some attendees were even seen to be sporting tattoos on their forearms to mark the occasion.

Jon Pratt of the US National Institute of Standards and Technology said the vote left him “extremely emotional”.

“Those units, those constants chosen now, include everything we know, everything we have always known and provide that springboard for us to go pursue those things that we don’t know,” he said.

In the measurements vote, countries also unanimously approved updates to three other key units: the kelvin for temperature, the ampere for electrical current and the mole for the amount of a substance.

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