On 19 March 1791 five of the great luminaries of French science, Laplace, Lagrange, Condorcet, and , met at the in Paris and drew up a document that … - Terence Quinn

In my house I have four , I sometimes adjust them so that at least two, but rarely more than three, strike the hour within a few seconds of each other. Fortunately, the pleasure I get from my clocks does not depend on them all telling the same time. When I switch on the in my car, however, things are different. It very quickly latches on to at least four atomic clocks high in the sky, all of which tell the same time to about a hundred millionth of a second. If they did not, my GPS would guide me to somewhere other than my desired destination.

It has long been recognized that the basic units in science – such as the and the – should be defined in terms of fundamental physical phenomena. Indeed in 1870 James Clerk Maxwell recognized that the units of , and would only remain unchanged and reproducible if they were defined by the , period of vibration and absolute mass of molecules rather than by the physical properties of the Earth. However, it took over a century for the metre and the second to be defined in terms of the quantum properties of atoms. And it was only in 1990 that reproducible standards of and were linked to quantum phenomena.

, the science of measurement, is part of the essential but largely hidden infrastructure of the modern world. We need it for high-technology manufacturing, human health and safety, the protection of the environment, global climate studies and the basic science that underpins all these. Highly accurate measurements are not exclusively the preserve of the and engineering; many areas of chemistry, and medicine are now dependent on accurate quantitative measurements. in all manufactured and agricultural products is strictly controlled by regulations that need accurate metrology for their implementation.