... of may allow us to say that "this object is hotter or colder than that one." But even this apparently simple statement is fraught with pitfalls f… - Terence Quinn

, 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.

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.

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 laid down the definition of the new basic unit of length, the , for the proposed new system of measurement that would become the .