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Why Is the New Kilogram Better? On the Revolution in the International SI System of Units and the Redefinition of the Mass Standard.

/edited by Prof. Tadeusz Szumiata, PhD, UTH Radom/

 

Why Is the Old Mass Standard Too Old?

The kilogram standard was the last in the entire international SI system of measures to be based on an artefact. It was a cylinder of platinum and iridium (90% Pt and 10% Ir) with a height equal to the diameter of about 39 mm. It was given the name Le Grand K ("The Great K"). This standard was kept at the Bureau of Weights and Measures BIPM (Bureau International des Poids et Mesures) in Sèvres near Paris and had been in force since September 1889, by decision of the First General Conference of Weights and Measures (Conférence Générale des Poids et Mesures, CGPM). Over the decades, science and technology have developed and revolutionised the metrology of various physical quantities, but the standard of the kilogram has remained unchanged. There was a lack of rational ideas about how to change the mass standards, but eventually it became apparent that Le Grand K began to show different trends in long-term mass changes compared to the comparison copies. The reason for these changes was the difficult to monitor phenomena of loss or absorption of atoms on the surface of individual standards.

Fig. 1. A replica of the existing international mass standard (Le Grand K) in Sèvres near Paris.(https://en.wikipedia.org/wiki/International_Prototype_of_the_Kilogram#/media/File:Prototype_kilogram_replica.JPG)

As reported by the world's most prestigious scientific journal, NATURE [1], over the century the mass difference between the superior standard (Le Grand K) and the reference copies has been as much as 50 micrograms (Figure 2).