# Evening Lecture: The SI Redefined

**“The SI Redefined”**

**Dr Mike de Podesta, National Physical Laboratory**

A talk about why measurement matters & the imminent redefinition of four SI units.* *Progress in science and engineering is often linked to progress in metrology – the science of measurement. If we cannot measure something then we cannot begin to understand it (science) or improve it (engineering). Measurement is the quantitative comparison of an unknown quantity with a standard. In the *International System of Units* – the ‘SI’ – there are seven standard quantities called the ‘base units’: the second, metre, kilogram, ampere, kelvin, candela and mole. The perfection with which these definitions can be made real, represents a fundamental limit to achievable measurement uncertainty. In order to remove these limits, from May 2019 there will be subtle but profound changes in the definitions of four of these base units – the kilogram, ampere, kelvin and mole. In this talk I will explain the rationale for the changes and why, even though you are unlikely to personally notice any changes, you should be happy about them.

Image Copyright: J.L. Lee / NIST