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Scientists to overhaul International System of Units

On Friday 16 November 2018, a vote will be made to redefine the world's definition of the kilogram, the kelvin, the mole and the ampere

  • si-units_2-(1).jpgThe 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)
  • 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)

Old definitions:

  • 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
  • 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
  • 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.

Notes to editors

About NPL

NPL is the UK's National Measurement Institute, providing the measurement capability that underpins the UK's prosperity and quality of life.

From new antibiotics to tackle resistance and more effective cancer treatments, to secure quantum communications and superfast 5G, technological advances must be built on a foundation of reliable measurement to succeed. Building on over a century's worth of expertise, our science, engineering and technology provides this foundation and helps to make the impossible possible. We save lives, protect the environment and enable citizens to feel safe and secure, as well as support international trade and commercial innovation. As a national laboratory, our advice is always impartial and independent, meaning consumers, investors, policymakers and entrepreneurs can always rely on the work we do.

Based in Teddington, south-west London, NPL employs over 600 scientists. NPL also has regional bases across the UK, including at the University of Surrey, the University of Strathclyde, the University of Cambridge and the University of Huddersfield's 3M Buckley Innovation Centre.

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About the BIPM

The signing of the Metre Convention in 1875 created the BIPM and for the first time formalised international cooperation in metrology. The Convention established the International Bureau of Weights and Measures and laid the foundations for worldwide uniformity of measurement in all aspects of our endeavours, historically focusing on and assisting industry and trade, but today just as vital as we tackle the grand challenges of the 21st century such as climate change, health, and energy. The BIPM undertakes scientific work at the highest level on a selected set of physical and chemical quantities. The BIPM is the hub of a worldwide network of national metrology institutes (NMIs) which continue to realize and disseminate the chain of traceability to the SI into national accredited laboratories and industry.

12 Nov 2018