"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."