The new SI based on fundamental constants.

The papers in this Discussion Meeting Issue of Philosophical Transactions A are based on the presentations given at a Discussion Meeting held at the Royal Society in London on 24 and 25 January 2011 entitled ‘The new SI: units of measurement based on fundamental constants’. The meeting was organized by the writer (Terry Quinn) in collaboration with Ian Mills FRS and Patrick Gill. The programme of lectures along with the biographies of lecturers and chairpersons can be found on the Royal Society website at http://royalsociety.org/new-SI/. The meeting was opened by the Minister for Universities and Science, the Right Honourable David Willetts MP. The PowerPoint presentations of all the lectures were subsequently placed on the website of the International Bureau of Weights and Measures (Bureau International des Poids et Mesures, BIPM) at www.bipm.org/en/si/new_si/discussion_meeting.html. The International System of Units (SI) was formally adopted by the 11th General Conference on Weights and Measures (Conférences Générales des Poids et Mesures, CGPM) in 1960 to meet the need for a comprehensive system of units suitable for the twentieth century. It was founded on six base units, the metre, kilogram, second, ampere, kelvin and candela. At the time, the metre and kilogram were both still based on artefacts, with the International Prototypes of the metre and kilogram being kept at the BIPM, Sévres, France. The ampere and candela were also linked to these artefacts because of the dependence of their definitions on those of the metre and kilogram. The second, previously taken to be the fraction 1/86 400 of the mean solar day, had only recently been redefined in 1956 as a fraction of the period of the orbit of the Earth around the Sun, the tropical year. The 11th CGPM in 1960, however, in addition to adopting the name SI (Système International d’Unités), redefined the metre in terms of the wavelength of the orange light of the krypton atom. By so doing, it took the first step towards a system of units that would meet the precept of James Clerk Maxwell, who famously said at the 1870 meeting of the British Association for the Advancement of Science (Maxwell, J. C. 1870 Report of the 1870 BA Meeting, Notices and Abstracts of Misc. Comm., Mathematics and Physics, pp. 1–9):