The SI Redefinition to Come into Force from 20 May 2019

The 19th Meeting of the Directors of National Metrology Institutes (NMIs) and Member State Representatives of the International Bureau for Weights and Measures (Bureau international des poids et mesures, BIPM) was held at its office at Sèvres, France during 18 and 19 October 2017. Before we discuss the major outcome of this meeting, it is worth understanding the International Convention on SI and derived units and formation of National Metrology Institutes (NMIs), which is schematically shown in Figure 1. On 20 May 1875, 20 industrially developed nations met for the ‘Convention of the Metre’ and signed the ‘Treaty of the Metre’ under which three bodies were set up to take custody of the international prototype kilogram and metre, and to regulate comparisons with national prototypes.