Logo of the Physikalisch-Technische Bundesanstalt
symbolic picture: "magazines"

New SI into force

PTBnews 2.2019

On 20 May 2019, World Metrology Day, the revised International System of Units (SI) entered into force. On 16 November 2018, the revision was adopted unanimously by the representatives of all member states of the Metre Convention at the 26th General Conference on Weights and Measures (CGPM) held in Versailles. Since 20 May 2019, all SI units are based on a set of 7 defining natural constants. Thus, the differentiation between base units and derived units de facto no longer applies. In everyday life and for any standard consumer, nothing has changed. The change is, however, relevant to precision metrology as well as to teachers who will no longer be able to tell their students the relatively simple story of the international prototype of the kilogram but will have to elucidate the far more complex universe of natural constants.