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