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> 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 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> A vote by scientists to redefine the kilogram in terms of universal natural constants has brought about confusion online, as people joked about metric system measurements that few seemed to fully understand.

For over 130 years, the kilogram was physically represented by one very special lump of platinum. On Friday, the General Conference on Weights and Measures voted to redefine the unit based on the amount of electric current required to counteract the mass, making the last of the International System (SI) units based on universal constants rather than physical objects.

If this feels like it’s going a bit over your head… you’re not alone.

While those in the US may be confused by the term kilogram, rest assured, it’s all part of that pesky metric system used by the entire world (except for the US, Liberia and Myanmar, that is). The US measures for weight – Imperial pounds and ounces – remain entirely unaffected.

The vote to change the kilogram spells the demise of “Le Grand K” (the Great K), a platinum weight that has been kept in a French vault since 1889.

This is something many scientists have long advocated changing in favor of something that’s more, well, scientific. So overjoyed were the scientists that a number of people reportedly burst into tears of joy when the unanimous vote came in.

Social media was awash with general weight-related puns as the news broke.

Unfortunately, this doesn’t mean you can expect to find yourself facing a different number when you step on the weighing scales, or that your luggage allowance will increase. The difference in what makes a kilo a kilo will not be felt by ordinary people, or drug dealers.

It also doesn’t mean that they’re coming for our other SI measurements, like the kilometer (we hope). Not that this stopped many social media users from cracking jokes about miles and inches, which – in case metric users around the world are unaware – measure distance, not mass.

Like this story? Share it with a friend! </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 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> 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> • Over 50 scientists have voted in a change which will see the 129-year-old method of defining a kilogram scrapped.
• The new method, decided at the General Conference on Weights and Measures in Paris on Friday, will be a formula called the “Planck constant.”
• A lump of platinum-iridium called “Le Grand K,” which only left its protective case four times in the last century, was used as the previous definition.
• They made the change as the “Grand K” was losing mass: equivalent to the weight of a eyelash.

Scientists in Paris on Friday approved a change which will alter how a kilogram is defined, the first change to the system of measuring mass which has been in use since 1889.

Scientists from over 50 countries voted to change the definition to a scientific formula called the “Planck constant (h),” which works out a kilogram based on the amount of electricity needed to counteract its force.

They met at the General Conference on Weights and Measures in Versailles, Paris.

The previous way scientists defined a kilogram was by the weight of a “platinum-iridium lump,” called “Le Grand K,” which is kept securely at a site just outside Paris, but will now be retired.

This lump was established as the singular measure for all kilograms in 1889 and has only left its protective case four times in the last 100 years, Sky reported.

But the lump, and the identical copies of it, experienced a minute amount deterioration according to Pallab Ghosh, the BBC’s science correspondent.

In fact the changes were so small they were about 50 parts out of a billion, or the weight of an eyelash. By definition, whatever the lump weighed was a kilogram, so if a chunk fell off, then the mass of all the matter in the universe, if measured by kilograms, would change.

Read more: Top mathematician says he solved the ‘single most important open problem’ in math after 160 years

The new method ditches a physical standard in an attempt to provide a “stable foundation from which to advance our scientific understanding, develop new technologies and address some of society’s greatest challenges,” Martin Milton, Director of the International Bureau of Weights and Measures (BIPM) told the National Physical Laboratory.

Perdi Williams from the National Physical Laboratory said: “The new system is going to work a lot better. It is also a really exciting time, and I can’t wait for it to happen,” the BBC reported.

At the event, organised by the International Bureau of Weights and Measures (BIPM,) several other units of measurement have had their definitions changed: the ampere, the kelvin, and the mole.

The ampere will be defined by the elementary electrical charge (e,) the kelvin will be defined by the Boltzmann constant (k,) and the mole will be defined by the Avogadro constant.
The redefinition of the SI system of measurement adopted by the majority of countries in the world takes effect.