Scientists from around the world are gathering in France today to decide the fate of the kilogram.

For nearly 130 years, the kilogram has been based on a lump of metal called the Big K, locked in a vault near Paris.

Key points: More than 60 countries will take part in a historic vote in Versailles to redefine the kilogram

More than 60 countries will take part in a historic vote in Versailles to redefine the kilogram Our unit for mass will no longer be based on the weight of the Big K, but on fixed value from nature: Planck's constant

Our unit for mass will no longer be based on the weight of the Big K, but on fixed value from nature: Planck's constant The conference will also formalise new definitions for three other units: the ampere for current, Kelvin for temperature and mole for molecules

But at 11:00am Paris time (9:00pm AEDT), representatives from 60 nations, including Australia, will vote on a proposal to define the humble unit using pure, unadulterated physics.

It's not a change that you'll notice when you go shopping, but it is necessary if we want to keep pace with technology now and into the future, said Australia's chief metrologist Bruce Warrington.

"There's been a dream for more than a couple hundred years to tie our units of measurement to fundamental properties of nature," he said.

"You want your system of measurement to be the best and most stable it can be."

The International Bureau of Weights and Measures, which governs changes to measurements, has already updated several of its units.

"We have atomic clocks that tell a second, and we use lasers using the speed of light to measure our metre, but the kilogram is still this 19th-century artefact," said Dr Warrington, CEO of the National Measurement Institute.

A replica of the international prototype kilogram in Paris, France. ( Wikimedia Commons: Japs88 )

History of the kilogram

Back in the 18th century a kilogram was equivalent to a certain volume of water, Dr Warrington explained.

But in 1889, it was replaced with a cylinder of platinum and iridium known as the international prototype kilogram (IKP) — or the Big K.

Today the original Big K is held under lock and key in the Pavillon de Breteuil, Saint-Cloud, near Paris.

There are six copies in the same vault, and many more around the world.

"At the same time as it was made, there were lots of copies made. We have copy number 44," Dr Warrington said.

Over time, there have been slow drifts between Big K and its copies.

A Kibble balance uses quantum mechanics to precisely measure an object's weight. ( Supplied: J. L Lee/NIST )

"We're only talking micrograms. Not enough to worry us in our daily life measurements, but enough to see, and enough to want a better kilogram," Dr Warrington said.

And that's important for international trade.

"Originally, if you were trading with your international partners, you needed to know that the wheat that went onto the boat was the same wheat that came off. And pressure is only bigger today with global supply chains.

"You need to know that an aircraft part made in Australia will bolt to the rest of the aircraft from around the world."

So what's going to change?

Instead of using Big K as a yardstick, the scientists are proposing to use one of the fundamental laws of nature known as the Planck constant to define a kilogram.

The Planck constant is the amount of energy released in light when electrons in atoms jump around from one energy level to another, explained physicist Tim Bedding of Sydney University.

"The committee is going to define the value of the Planck constant to be a particular number," Dr Bedding said.

That number will now be exactly 6.62607015 x 10^-34 Joule seconds.

"Therefore, we'll use that to define the kilogram — in the same way that the speed of light (299,792,458 metres per second) is fixed and we use that to define the metre," Dr Bedding said.

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To make their measurements, scientists will use a sensitive electromagnetic instrument known as a Kibble balance.

"Basically, it's a very precise measurement of how much electricity is needed to run through a wire in order to support the mass of an object," Dr Bedding said.

"It's enormously expensive, so people aren't just going to have a Kibble balance lying around, but the point is it's more reliable and it's going to last longer than a lump of material.

"I think it's quite interesting that basically they're saying that we believe the laws of physics so much that we're willing to redefine the kilogram in terms of a physics experiment."

But wait there's more…

At the same meeting in Versailles — the 26th General Conference on Weights and Measures — delegates will also vote to redefine three other units.

The elementary charge of an electron will be used to quantify the ampere: the basic unit of electrical current.

"An ampere is a flow of charge per second," Dr Bedding explained.

The Kelvin, a unit of temperature, will be measured by defining the Boltzmann constant.

"That essentially tells you how much energy a material has at a certain temperature," Dr Bedding said.

And the mole, a unit that refers to a quantity of atoms or molecules, will now be defined by setting a specific number for the Avogadro constant.

Until now the mole has been tied to the kilogram, Dr Warrington said.

"It was the number of atoms in a certain quantity of carbon, measured in kilograms," he said.

The revised SI units The second is the time taken for a certain number of transitions in the state of a caesium 133 atom, which happens at a fixed frequency of 9,192,631,770 Hz The metre is the distance travelled by light in a specific fraction of a second, defined using the speed of light in a vacuum of 299,792,458 metres/second The Kelvin is the unit of temperature, defined using the Boltzmann constant for the energy of particles of gas at a given temperature, set at: 1.380 649 × 10 –23 Joules/Kelvin

The is the unit of temperature, defined using the Boltzmann constant for the energy of particles of gas at a given temperature, set at: 1.380 649 × 10 Joules/Kelvin The kilogram is a quantity of mass, defined using the Planck constant set at: 6.626 070 15 × 10 –34 Joules-second

The is a quantity of mass, defined using the Planck constant set at: 6.626 070 15 × 10 Joules-second The ampere is a flow of current, quantified using the elementary charge of electrons set at: 1.602 176 634 × 10–19 Coulomb The candela measures light intensity and is defined using the constant K cd : the brightness, per Watt, of monochromatic radiation at a specific frequency (540 × 10 12 Hz) — namely 683 lumen/Watt

The measures light intensity and is defined using the constant K : the brightness, per Watt, of monochromatic radiation at a specific frequency (540 × 10 Hz) — namely 683 lumen/Watt The mole is a quantity of atoms or particles, defined by the Avogadro constant set at: 6.022 140 76 × 1023 mol–1

When the vote passes, the kilogram, Kelvin, ampere and mole will fall in line with the other three International Systems of Units (SI), which are already defined in terms of fundamental constants: the metre, second and candela — the unit of luminous intensity.

"It's an historic step, it's bigger than any similar change made in the past," Dr Warrington said.

Speaking to RN Breakfast, he explained it would future-proof our system of units.

"When you fix the value of a fundamental constant, as your technology evolves you can make better and better measurements into the future. That's what we need to take advantage of new technologies, new industries, new products."

And it's an ambition, he added, that dates back to the French Revolution, for "measurement that's accessible to anyone".

Dr Warrington will be casting Australia's vote at the conference and he is confident the changes will go through.

"Everybody's been building towards this new agreement for such a long time that we're all looking forward to it," he said.

"We don't know for sure what the innovations of tomorrow are going to come out of the kilogram, but history tells us they will be amazing."

If the changes are accepted, they will come into force on the next World Metrology Day: May 20, 2019.