The definition of four of the SI units: kilogram, mole, ampere and the kelvin are set to change

The definition of four of the SI units: kilogram, mole, ampere and the kelvin are set to change All International System of Units (SI) will be based on fundamental constants of nature, providing stable foundation for the future of science

UK scientists have been responsible for many of the developments that now enable redefinition

On Friday 16 November 2018, measurement scientists from around the world will come together to witness a vote on the redefinition of the International System of Measurement (SI) units, changing the world's definition of the kilogram, the kelvin, the mole and the ampere, for ever.

Taking place at the General Conference on Weights and Measures in Versailles, Paris, hosted by the International Bureau of Weights and Measures (BIPM), this decision will mean that all of the units are expressed in terms of constants that can be observed in the natural world.

The redefinition would bring an end to physical artefacts like the kilogram, which is currently defined as equal to the mass of the International Prototype of the Kilogram (a block of metal stored in a vault in France). This artefact is susceptible to damage and environmental factors, and is compared to its copies only once in every 40 years, making calibration to it difficult and potentially inaccurate.

This decision will ultimately lead to a more practical definition of the SI. Using unchanging standards as the basis for measurement will mean that they will remain reliable into the future. Just as redefining the second and the metre helped enable GPS navigation, the redefined SI is expected, over time, to enable new technologies we have yet to even imagine, whilst maintaining continuity for practical users.

The National Physical Laboratory (NPL), the UK's National Measurement Institute, which is responsible for measurement standards across the country, has been an international leader in the global effort to achieve redefinition. The second was redefined by the clock that Louis Essen developed at NPL, and NPL's work has played vital roles in enabling changes to the kelvin, ampere and mole. The Kibble balance, the instrument that measures the Planck constant – the natural constant that the kilogram will be defined by – was developed by the late NPL scientist Dr Bryan Kibble.

"The SI redefinition is a landmark moment in scientific measurement," said Dr JT Janssen, Director of Research at NPL. "Once implemented, all the SI units will be based on fundamental constants of nature whose value will be fixed for ever. This will pave the way for far more accurate measurements and lays a more stable foundation for science."

If approved, as is expected, the redefinition will come into effect on World Metrology Day, 20 May 2019.

In total, the definitions of four of the seven base SI units will be redefined. This will not only impact on scientific discovery and innovation, but industry and everyday society – with wide-reaching consequences in technology, retail, health and the environment, among many other sectors.

The expected new definitions:

The kilogram – will be defined by the Planck constant (h)

– will be defined by the Planck constant (h) The ampere – will be defined by the elementary electrical charge (e)

– will be defined by the elementary electrical charge (e) The kelvin – will be defined by the Boltzmann constant (k)

– will be defined by the Boltzmann constant (k) The mole – will be defined by the Avogadro constant (N A )

Old definitions:

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram

is the unit of mass; it is equal to the mass of the international prototype of the kilogram The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length

is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length The kelvin is the unit of thermodynamic temperature, which is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water

is the unit of thermodynamic temperature, which is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

12 Nov 2018 </s> On 16th November 2018, measurement scientists from more than 60 countries will come together to witness the vote on the redefinition of the International System of Units (SI), changing the world’s definition of the kilogram, the ampere, the kelvin and the mole. If agreed, these changes will come into force on 20th May 2019. The vote will take place at the General Conference on Weights and Measures in Versailles, France, which is organized by the International Bureau of Weights and Measures (BIPM). The decision will mean that all SI units will be defined in terms of constants that describe the natural world. This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.

The decision will bring an end to the use of physical objects to define measurement units. The kilogram, for example, is currently defined as being the mass of the International Prototype of the Kilogram, a cylinder of a platinum alloy stored at the BIPM in France. This object has served science and technology well for nearly 130 years, but its stability during this period could only be confirmed by comparisons with identical copies, which is a difficult process and potentially inaccurate. The proposed change will see the kilogram defined by the Planck constant – the fundamental constant of quantum physics.

“The SI redefinition is a landmark moment in scientific progress.” said Martin Milton, Director, International Bureau of Weights and Measures (BIPM), “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.”

Julian Braybrook, Government Chemist, said: “In many of the chemical sciences, the concepts and application of measurement science are a largely accepted practice. The redefinition of some of the SI may therefore seem significant but in common practice, little will change as a result. A system based on the fundamental constants of nature, rather than potentially changing physical artefacts such as the kilogram, will ensure a more stable basis for the accurate and safe measurement of quantities of food ingredients or contaminants, the premise of which underpins discharge of the statutory duties of the Government Chemist.”

The expected new definitions impact four of the seven base units of the SI: the kilogram, ampere, kelvin and mole, and all units derived from them, such as the volt, ohm and joule.
• The kilogram – will be defined by the Planck constant (h)
• The ampere – will be defined by the elementary electrical charge (e)
• The kelvin – will be defined by the Boltzmann constant (k)
• The mole – will be defined by the Avogadro constant (NA)

Although the size of these units will not change (a kilogram will still be a kilogram), the four redefined units will join the second, the metre and the candela to ensure that the revised SI will maintain its relevance by facilitating technical innovations. Just as the redefinition of the second in 1967 provided the basis for technology that has transformed how we communicate across the globe, through GPS and the internet, the new changes will have wide-reaching impact in science, technology, trade, health and the environment, among many other sectors.

For more information about the work the Government Chemist does contact: </s> A replica of the International Prototype Kilogram is pictured is seen at the 26th meeting of the General Conference on Weights and Measures (CGPM) to vote on the redefinition of four base units of the International System of Units (SI) in Versailles, France, November 16, 2018. REUTERS/Benoit Tessier

PARIS (Reuters) - It may not change how you buy bananas, but scientists have voted to redefine the value of a kilogram, in what they called a landmark decision that will boost the accuracy of scientific measurements.

Since 1889, a kilogram has been defined by a shiny lump of platinum-iridium kept in a special glass case and known as the International Prototype of the Kilogram. It is housed at the headquarters of the International Bureau of Weights and Measures (whose French acronym is BIPM), just outside Paris.

Members of the BIPM, which groups some 60 nations, agreed on Friday after a week-long meeting at the nearby Palace of Versailles to redefine a kilogram in terms of a tiny but unchanging value called the "Planck constant".

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

All modern mass measurements are derived from the kilogram, whether micrograms of pharmaceutical medicine or gold dust, kilos of fruit or fish, or tonnes of steel.

The problem is the prototype 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.

Scientists have been trying for decades to define a constant value for the kilogram that is derived from immutable physics, in the same way they have done for other standard units (SI units) overseen by the BIPM.

For example, a meter isn't 100 centimeters, it's actually "the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second".

The "Planck constant", which derives from quantum physics, can be used along with a Kibble balance, an exquisitely accurate weighing machine, to calculate the mass of an object using a precisely measured electromagnetic force.

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

"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.

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.

The new definitions agreed by the BIPM will come into force on May 20, 2019. </s> New Delhi, May 20 (IANS) The new System of Units (SI) to measure weight, temperature, amount of substance, and current came into operation in India from Monday on the occasion of the World Metrology Day, an official release said.

The resolution to redefine four of the seven base units – the kilogram (SI unit of weight), Kelvin (SI unit of temperature), mole (SI unit of amount of substance), and ampere (SI unit of current) – was adopted at the 26th General Conference on Weights & Measures (GCWM), which is comprised of 60 member countries, in November 2018.

“This decision has now enabled scientists and researchers to base the SI units entirely on fundamental properties of nature, which will ensure their ongoing refinement and improvement for years to come,” the Science and Technology Ministry Ministry said in a release.

“The fundamental constants are invariants of time and space and successfully replaced the artifact-based units, and aptly opened up the new era for quantum world by linking all seven base units to fundamental constants/quantum standards.”

The existing definition of the kg is over 130 years old. The new SI system, which is defined in terms of Planck’s constant, would be stable in the long term and practically realisable.

The new SI System will be helpful in bringing in accuracy while dealing with international trade, biotechnology, high-tech manufacturing and human health and safety.

The new definition of kg involves accurate weighing machines called ‘Kibble balance’, which uses Planck’s Constant to measure the mass of an object using a precisely measured electromagnetic force. </s> The weight is finally over. Nearly 130 years after the kilogram was first defined by a lump of metal in a vault in Paris, scientists have voted for change and a new system that redefines the global measure of mass in terms of a fundamental constant of nature.

Following a historic vote on Friday at the General Conference on Weights and Measures in Versailles, the kilogram will no longer be defined by the international prototype kilogram (IPK), a platinum alloy cylinder fashioned in 1889, but by Planck’s constant, a number that is deeply rooted in the quantum world.

“The vote went through unanimously. It was very emotional,” said Stephan Schlamminger, a physicist from the US National Institute of Standards and Technology who has worked on redefining the kilogram for years. “They did a roll call of each country. One got a sense of how big metrology is.”

The IPK, or Le Grand K, has served the world well but scientists have long known that even though, by definition, it is precisely one kilogram, its weight changes over time. When in use, the tiniest wear makes it lighter, while pollution in the air binds to the surface and over time makes it ever so slightly heavier.

The new system retires the IPK and instead defines the unit of mass through the electrical force needed to counteract the weight of a kilogram on a machine called a Kibble balance. The electrical force itself is linked to the Planck constant through quantum electric effects described by two Nobel prize winners, Brian Josephson and Klaus von Klitzing.

Along with the kilogram, three other base units will be redefined as a result of the vote. The units for electric current (ampere), temperature (kelvin) and amount of substance (mole) all become linked to constants of nature, namely the electric charge, the Boltzmann constant and the Avogadro constant respectively. In all, the International System of Units has seven base units, including the unit of time (second), the unit of length (metre), and the unit of luminosity (candela). The definitions of these will remain unchanged.

Martin Milton, director of the International Bureau of Weights and Measures, said the redefinition was “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,” he said.

While the redefinition will have zero impact on the way fruit and vegetables are weighed at the supermarket, it marks the culmination of decades of work to link the basic units that underpin metrology to constants woven into the fabric of the universe. The changes will come into effect on 20 May 2019, world metrology day. </s> How much is a kilogram? 1,000 grams. 2.20462 pounds. Or 0.0685 slugs based on the old Imperial gravitational system. But where does this amount actually come from and how can everyone be sure they are using the same measurement?

Since 1889, countries who are members of the General Conference on Weights and Measures have agreed to use a standard block of metal – kept near Paris – to define the kilogram. But although the modern block is stored in a highly controlled environment, its weight can change by tiny amounts as wear and tear causes it to lose mass and dirt causes it to increase. To address this problem, scientists around the world have spent nearly two decades discussing how the kilogram could instead be defined in relation to constant measurements of nature. And now they have finally reached a decision.

The first kilogram (originally called a grave) was defined in 1793 by a commission of the French Academy of Sciences, who wanted a better standard than the fixed amounts of grain that had traditionally been used. The commission decided that the new measure would be the mass of one cubic decimetre of distilled water at 4℃ (the temperature at which water has its highest density under standard conditions). This had the advantage in that most properly equipped labs would be able to reproduce this standard. Subsequently, a prototype of this mass was cast in brass.

Unfortunately, this definition of mass depended upon another variable measurement, the metre. At this point, the metre was only provisionally defined as part of the distance from the North Pole to the equator. Once the value of the metre and the temperature of water at its densest were more accurately defined, the kilogram also had to be replaced. And a new prototype was cast in platinum to represent this mass.

Eventually, this was replaced with the international prototype kilogram or IPK used today, cast from a mixture of platinum and iridium to make it very hard and prevent it reacting with oxygen. The IPK and six copies are kept by the International Bureau of Weights and Measures in the Pavillon de Breteuil, Saint-Cloud, near Paris in France to act as a reference to measure against. Copies of the IPK are transported across the world to ensure all participating countries use the same standard.

But even the modern IPK can gradually change in mass. Radically, the answer from the International Bureau of Weights and Measures is to overhaul the definitions of a kilogram, as well as all the other basic units of measurement used in science (known as SI units, from the French for international system).

Instead of measuring the kilogram against a block stored in a vault, we can define it based on precise values of constants of nature. Agreeing a definition has taken a long time because we needed to be able to measure these constants to exacting standards with an uncertainty of 30 parts per billion (meaning the measurements are accurate to 0.00000003 of a unit).

Scientists have actually already done this for time and length. A second is no longer a fraction of the time it takes the Earth to revolve, which can change as the globe speeds up or slows down. Instead, a second is now defined by the time it takes for a certain amount of energy to be released as radiation from atoms of Caesium-133. Specifically, one second equals 9,192,631,770 transitions in the hyperfine ground state levels of Caesium-133. This is the same no matter when or where it is measured.

Scientists were then able to redefine the metre in relation to the second and another natural constant, the speed of light in a vacuum (c), which scientists have calculated as 299,792,458 metres per second. So one metre is now the length travelled by light in 1/c seconds.

The new definition of the kilogram uses a measurement from another fixed value from nature, Planck’s constant (h), which will be defined as 6.62607015×10−34 joule seconds. Planck’s constant can be found by dividing the amount of energy a particle of light or “photon” carries with its electromagnetic frequency.

The constant is usually measured in joule seconds but this can also be expressed as kilogram square metres per second. We know what a second and a metre is from the other definitions. So by adding these measurements, along with an exact knowledge of Planck’s constant, we can get a new, very precise definition of the kilogram.

Part of the reason creating the new definition has taken so long is because scientists have had to create very precise devices to measure Planck’s constant with a high enough degree of accuracy. The method has also been controversial because it will break the link that the kilogram has to other base SI units, particularly the mole, which measures the amount of a substance in terms of the number of particles it’s made from. Some scientists have proposed alternative methods as a result.

But following a symbolic vote, the new definition of the kilogram will be used by the International Bureau of Weights and Measures and national measurement institutes around the world, along with new definitions of the remaining base SI units, the mole, the kelvin (temperature), the ampere (current) and the candela (luminous intensity).

For most people, everyday life will carry on as normal despite the redefinitions. One standard bag of sugar will contain as much sugar as it ever did. But some of these changes, for example to the kelvin, will mean practical advantages for scientists making very precise measurements. And to answer the question “how much is a kilogram”, we will no longer have to compare blocks of platinum or worry about scratching them.

This article first appeared on The Conversation. </s> The recommended practical system of units of measurement is the International System of Units (Système International d'Unités), with the international abbreviation SI.

The SI is defined by the SI Brochure, which is published by the BIPM.

In a landmark decision, the BIPM's Member States voted on 16 November 2018 to revise the SI, changing the world's definition of the kilogram, the ampere, the kelvin and the mole.

This decision, made at the 26th meeting of the General Conference on Weights and Measures (CGPM), means that from 20 May 2019 all SI units are defined in terms of constants that describe the natural world. This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.

The seven defining constants of the SI are:

the caesium hyperfine frequency Cs ;

; the speed of light in vacuum c ;

; the Planck constant h ;

; the elementary charge e ;

; the Boltzmann constant k ;

; the Avogadro constant N A ; and

; and the luminous efficacy of a defined visible radiation K cd .

The SI was previously defined in terms of seven base units and derived units defined as products of powers of the base units. The seven base units were chosen for historical reasons, and were, by convention, regarded as dimensionally independent: the metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela. This role for the base units continues in the present SI even though the SI itself is now defined in terms of the defining constants above. </s> The U.S. National Institute of Standards and Technology called this Friday a “turning point for humanity,” even though the difference between this day and the day before weighs, theoretically, nothing at all.

But, while something that weighs a kilogram — the basic unit of mass — is just as heavy as it was before, the way that kilogram is defined has changed, after a vote by scientists at the General Conference on Weights and Measures in Paris.

For 130 years, the kilogram has been defined as the exact weight of a cylinder of platinum-iridium held in a triple-locked vault in Paris. Metrologists — experts in the science of measurement — refer to the metal lump as the International Prototype Kilogram, or IPK. Others call it “Le Grand K.” It is the one true kilogram in the world, upon which all measures of mass on Earth depend.

But that’s now changed. On Friday, representatives from 60 countries decided to redefine the kilogram, setting its weight in terms of electric current rather than by comparison to a set object. The decision means that all of the seven base units — the meter for distance, the second for time, the kilogram for mass, the mole for amount of substance, the ampere for electrical current, the kelvin for temperature and the candela for luminosity — are now defined in terms of fundamental constants that describe the natural world.

The change, which will come into force in May 2019, will draw the curtain on the use of human-scale physical objects to define measurement units, and thus end an important era in the history of science.

For most of human history, weights and measures were a local matter, often imprecise and varied. The kilogram was born from an effort to change that situation, in the late 18th century. In 1791, the French National Assembly realized that something had to be done about the chaotic state of French weights and measures. Their aim, redolent of revolutionary zeal, was to create a measurement system “for all time, for all people.”

The Assembly decided that a single, immutable system of weights and measures should be established — not that immutability would be the same thing as simplicity. Their new unit of length, the meter, was set at one ten-millionth of the distance from the North Pole to the equator, measured around the poles of the meridian passing through Paris. The liter, at one thousandth of a cubic meter, would be used to measure capacity. And the kilogram, which would measure mass, would equal the mass of one liter of water.

And so the metric system was born.

Not that it was widely used at first. More than three-quarters of a century passed after its official adoption in France before dignitaries from 17 countries signed the international Treaty of the Meter in 1875, thus harmonizing systems of measurement across the world. Fourteen years later, the IPK was built and then whisked away to a lab in Sèvres, France, where it has been kept ever since.

For 129 years, “Le Grand K” remained the reference point for measurements of mass around the world. But change was afoot.

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In 1983, the meter was redefined as the distance traveled by light in a vacuum in 1/299,792,458 of a second, and it was again redefined in 1997 as the duration of 9,192,631,770 natural oscillations of microwave radiation released by the element cesium.

And in 1989, the perils of using human-dependent standards of measurement, rather than those ultra-precise mathematical standards, were handily illustrated. It was discovered, to the dismay of metrologists everywhere, that the IPK was 50 micrograms lighter than the replicas of it that had been distributed around the world. Since “Le Grand K,” by definition, is a kilogram, any change to its mass necessarily alters the definition. Either “Le Grand K” had lost weight, or the replicas had been piling on the (micro)pounds.

Though 50 micrograms might not sound like much, even such a tiny change meant that the weight was no longer accurate enough for advanced research and technological applications. The question, then, was how to figure out a more fundamental — and thus more reliable and precise — way to define the mass of a kilogram.

Tortuous History of the U.S. and the Metric System

The new definition of a kilogram will be based on three fundamental constants: the speed of light; the cesium atom’s natural microwave radiation; and the Planck constant, which describes the size of the packets of energy that atoms and other particles use to absorb and emit energy.

The current kilogram mass exerts a specific amount of force in Earth’s gravity — in other words, it weighs something. The revised definition replaces this familiar determination of mechanical force with an electromagnetic measurement tied to the Planck constant and based on electrical current and voltage. Using an instrument called a Kibble balance, after its inventor Bryan Kibble, an electric current is generated in a coil to produce a magnetic field strong enough to offset a mass of one kilogram.

“This redefinition is a landmark moment in scientific progress,” said Martin Milton of the International Bureau of Weights and Measures (BIPM), in a statement. “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.”

“Today marks the culmination of decades of work by measurement scientists around the world, the significance of which is immense,” said Barry Inglis, Director of BIPM. “We will now no longer be bound by the limitations of objects in our measurement of the world, but have universality accessible units that can pave the way to even greater accuracy, and even accelerate scientific advancement.” </s> Deep underneath the Parisian suburb of Saint-Cloud, in a vault that can only be opened by three people wielding three different keys, there lies a hunk of metal that is so crucial to the world of measurements that it only ever leaves its tightly-controlled environment to be cleaned and weighed.

For the last 129 years, this small cylinder of platinum-iridium has defined the weight of the kilogram. The International Prototype of the Kilogram, or IPK, does not just weigh one kilogram. It is one kilogram. If its mass goes up or down – and in the past century it has done both – then the definition of a kilogram goes up or down too.

Such is the importance of the IPK, there is even a sixteen-page manual devoted to the task of cleaning the mass. First, it’s wiped with a chamois leather that has been soaked for 48 hours in a mixture of ethanol and ether to remove any impurities. It’s then sprayed with steam from double-distilled water. The remaining water is then blotted away with filter paper, or blown off with clean gas, before the IPK is flipped over and the process starts again.

In total, it takes 50 minutes to clean the IPK, which measures just 39 millimetres in both height and diameter. “It's the world's mass standard and you don't muck around with it, it's really rather important,” says Ian Robinson, a researcher at the National Physical Laboratory who has been working on the redefinition of the kilogram for nearly four decades.

But now the IPK’s reign is almost at an end. Today, delegates at the General Conference on Weights and Measures – an international conference that meets every six years to decide on reforms to to the metric system – are expected to vote to stop the IPK being used as the definition of the kilogram. Instead, the delegates from 40 different countries will opt to base the definition of the kilogram on physical constants. Last-minute rebellions in the cautious world of international measurements standards are generally unheard of, says Richard Brown, the NPL’s head of metrology.

So what’s wrong with the IPK? The problem, Brown explains, is that tying a definition to a physical object is nowhere near as useful as using physical constants. “Ultimately the traceability of the kilogram is through this single material artefact which is held at this international facility,” he says, but the IPK’s mass actually fluctuates by millionths of a kilogram as it accrues grime or loses mass when it gets cleaned. The aim of the conference is to switch the definition to something that can never change.

For most people, this precise definition of the kilogram isn’t going to change how we measure out flour or weigh ourselves, but for organisations such as the NPL, which promote accurate measurements across industries, getting precise about the kilogram is a huge deal.

All countries that are part of the Metre Convention – the 1875 treaty that established the created the international Bureau of Weights and Measures (known by its French initials, BIPM) – have their own copies of the IPK. Every 40 years these copy prototypes are carried – usually by hand – to Paris, where they are weighed to see how they compare to the IPK. The last weighing-in happened in 2014 to lay the groundwork for the upcoming redefinition, but there had only been three prior calibrations before then.

The UK’s copy is called Kilogram 18, and rests in a safe at the NPL’s headquarters in Teddington, South London. “It’s got a pretty nice life,” says Robinson, who, despite leading work on the redefinition of the kilogram, rarely works with Kilogram 18 itself. Like the IPK, Kilogram 18 – so-called because it was the 18th copy to be alloted by ballot and distributed – is kept under two bell jars filled with filtered air.

It’s these copies that national measurement laboratories, such as the NPL, use to set measurement standards and correctly calibrate delicate instruments. But the problem is that no copy is exactly the same mass as the IPK. Kilogram 18 is around 60 micrograms heavier than the IPK – overweight by an amount equivalent to a couple of small grains of sands. And that’s before you account for fluctuations in the mass of the IPK itself, as it picks up pollutants from the air or loses some of its alloy during cleaning.

“You're looking at stability of the mass, and in no circumstances can you get a mass that's exactly one kilogram, they all drift slightly,” says Robinson. “So what you're bothered about is that your own [mass] is the same difference as the IPK or you can predict that difference.”

This is a problem when you’re setting standards for industries such as pharmaceuticals, which regularly deal with measurements right down to micrograms. All metric mass measurements are derived from the kilogram and it’s extremely tricky to put in an accurate shot when your goalposts are moving every forty years or so. And that’s why Robinson and his colleagues have been working on a device that will allow scientists to leave behind the IPK for good.

Nearly 40 years ago, Robinson started working with his colleague, the late Bryan Kibble, on an instrument called a Kibble balance – a complex mess of scaffolding and wires that from a distance looks like it could be a spacecraft instead of an instrument that’s redefining one of our most important units of measurement.

Originally called the watt balance, but renamed the Kibble balance after Bryan Kibble’s death in 2016, the device was intended to solve a rather arcane problem with how the ampere – a unit that measures the flow of electrical current – is calculated. In 1988 Robinson and Kibble published a paper showing it was possible to use the Kibble balance to fix the definition of the ampere without having to use a complex instrument composed of two parallel lengths of wire.

Soon cracking the ampere, Robinson and Kibble realised that they could use the same device to leave the IPK in the dust. It could let them redefine the kilogram itself.

Here’s how it works. First you need to know that when you pass a current through a coil of wire, it generates a magnetic field. That’s exactly how loudspeakers work – they use electrical signals to switch an electromagnet on and off, causing a speaker cone to vibrate. What Kibble and Robinson realised is that you could balance this magnetic force against a physical mass – a little like resting a mass on your speaker cone and measuring how much electrical current you need to run through your electromagnetic to get it to move.

Through the clever deployment of a couple of physical laws – including one that won its creator the 1985 Nobel Prize in physics – the pair worked out that it was possible to express mass in terms of electromagnetic forces. Armed with this information, the pair worked out they could use the Kibble balance to calculate the Planck constant – an important number in quantum physics that relates to the amount of energy carried in a single particle of light.

Since Kibble and Robinson knew that they could calculate the Planck constant by starting with a mass, they also knew they could work backwards and derive a mass by starting with the Planck constant. And, unlike the IPK, the Planck constant does not fluctuate with time.

“You can then use any equation you like in physics to get yourself from the Planck Constant to mass,” says Robinson. “If you have the value of the Planck constant you can get a milligram, a kilogram, an atomic mass anything you like.”

If today’s vote goes ahead as expected, on May 20, 2019, the International System of Units – the system that sets the terms of metric measurements – will be updated to express the Kilogram in terms of the Planck constant.

But the kilogram isn’t the first of the seven base metric units to switch from a physical measure to a mathematical constant. A metre used to be defined as one ten millionth of the distance between the North Pole and the Equator. In 1983, the BIPM voted to instead derive the metre from the distance that light travels through a vacuum in a fraction of a second.

The kelvin, too, is getting an upgrade. Currently defined as a fraction of the temperature at which water can exist as a liquid, solid and gas, it will soon be defined in terms of the Boltzmann constant – a number that relates to the energy of particles in gas. It will join the kilogram along with the ampere and mole as the latest metric units to be defined in terms of fundamental constants.

If everything goes to plan, Brown says, no one outside of the scientific world will notice a difference when the kilogram switch is made next year. “It's very important that when we make the change, actually nothing changes,” he says. “We have to be absolutely sure that we're going to replace it with something that's better and something that is effectively the same size.”

That’s why Kilogram 18 is heading back to Paris for a final session to compare it with the IPK in preparation for the switch over to the Planck constant. After that, standards will be derived from Kibble balances instead of from kilogram prototypes. This will spur a whole new industry of people working to refine the Kibble balance, Brown hopes.

For Robinson, who has spent nearly four decades working on the device that will replace that IPK, today’s vote will be the conclusion of an extremely long and complicated process. After all, when you’re redefining one of the world’s most fundamental units of measurements, people tend to move extremely slowly.

“We were aiming at the kilogram in 1988-90, it's just that it's an extraordinarily difficult job,” Robinson says. But now, as he sets on his new task helping the world’s measurement bodies getting to grips with the Kibble balance, he’ll do so with the knowledge that behind the scenes of every weight measurement made in metric, his elegant solution to the kilogram problem is being put into flawless motion.

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The redefinition of the SI system of measurement adopted by the majority of countries in the world takes effect.