Abstract：Since the decimal metric system was introduced after the French Revolution, units of measurement have often been defined in terms of artefacts or properties of the earth or liquid water. The quest for truly universal definitions of units has led to the gradual adoption of fixed values of fundamental constants; the value of the speed of light was fixed in 1983 to define the metre. In 2018, a further revolution will take place with new definitions of the kelvin, kilogram, ampere and mole in terms of fixed values of the Boltzmann constant k, Planck constant h, fundamental charge e and Avogadro’s number NA. The seminar will chart the evolution of some of the base units, the advantages and shortcomings of their definitions and show how the present system will change in the near future. Perspectives for a future definition of the second will also be mentioned.

Brief Biography：Mark Plimmer is a lecturer at the Conservatoire national des arts et métiers, Paris, France. Born in England in 1964, he obtained his BSc. from Birmingham University in 1985. His doctorate at Oxford University (1985-89) was on laser spectroscopy of atomic systems (hydrogen, gadolinium, xenon). During postdoctoral work at Yale University and Ecole Normale Supérieure, Paris, he worked on measurements of the 1S Lamb shift in hydrogen and a determination of the Rydberg constant. After a further postdoctoral position at ENS studying parity violation in atomic cesium (1993-95), he took up his present post. From 1996-2004 he led a project to develop an optical clock based on a narrow two-photon transition in silver. During a spell in Switzerland (2004-07), he worked on the development of continuous cold atom cesium fountain clock. He was also involved in laser spectroscopy of N6+ using an electron beam ion trap via visits to Oxford University. Since rejoining the Cnam in 2007, he has worked with both the mass group where he studies surface effects using photothermal deflection, and the thermometry group working on a determination of Boltzmann’s constant. Since 2009, he has collaborated with the National Physical Laboratory, UK, on a new determination of the Rydberg constant.