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The New System of Units (SI)

The International System of Units, abbreviated as “SI” (French: Système international d’unités), is the global system for defining physical quantities. The "SI" is a metric system of units, which means one of the base units is the meter. After more than two centuries of constant efforts, the redefinitions of the base units ampere, kelvin, and mole have been put into effect on 20 May 2019, the World Metrology Day. The meter and the candela had already been defined earlier by physical constants. The last condition to be met was that the previous definition of the unit of mass, based on the kilogram artifact, could also be replaced.

The International Prototype of the Kilogram (IPK), approved by the international General Conference on Weights and Measures (CGPM) in 1889, was the worldwide reference for the kilogram. It is stored in the shape of a cylinder of platinum-iridium under a protective triple glass bell in the vault of the basement of the International Bureau of Weights and Measures (BIPM) in Sèvres near Paris. The problem with a definition such as "The kilogram is equal to the mass of the international prototype kilogram" is that the weight of this kilogram artefact has changed over the years. Even if these changes are very small, their causes and scale are incomprehensible, as the prototype kilogram, by definition, always weighs exactly one kilogram – no matter how heavy it actually is. With the continuous development of science and technology, maintaining the status quo was no longer acceptable. It was time to replace this artifact and instead derive the kilogram from physical constants.

Therefore, the definition of the International System of Units (SI) is now based exclusively on the fixed numerical values of seven defining constants. This new, "unalterable base" will fundamentally change metrological possibilities. At the same time, new standards and guidelines (such as DIN EN ISO/IEC 17025; IATF 16949) ensure that industrial processes are more closely linked to them.

The SI redefinition allows even more precise measuring equipment and, consequently, new future applications. At the same time, industrial development and the resultant higher accuracy requirements were also reasons why metrologists considered a new SI to be indispensable. Thus the circle is closing and metrology and industry can achieve a new level of "quality".

Written by: Dr. André Schäfer

The historical context can be found in the specified source:

"Nature is universal, so that discoveries made in London are equally valid in Paris, in Washington, or on the moon" Terry Quinn 

The author of "From Artefacts to Atoms"

Terry Quinn, Retired Emeritus Director of the Bureau International des Poids et Mesures (BIPM). 

Mr. Quinn was Director of BIMP from 1998 to 2003. He played a leading role in the establishment of the global Mutual Recognition Arrangement in measurement, which enabled international recognition of national measurement capabilities in support of innovation and trade. 

Ordering Information

Terry J Quinn "From Artefacts to Atoms - The BIPM and the Search for Ultimate Measurement Standards", published by Oxford University Press, New York 2012, ISBN: 9780195307863