The current definition of the kelvin introduced in 1954 depends on the temperature of the triple point of water, a substance obviously spread all over the globe but still very special, if low uncertainties are requested. Recent investigations revealed significant influence of the isotope ratio and of accidental contaminations on the temperature stability and reproducibility of the water triple point.

Therefore, a joint effort has been started among several NMIs to replace the current definition by referring it to a fundamental constant as already performed with the metre defined by fixing the speed of light in vacuum a couple of years ago.

As a first step, the Boltzmann constant, which is the conversion factor between the thermodynamic temperature and the thermal energy of a sample, has to be determined more precisely and by several different experimental approaches. Among these approaches acoustic thermometry in noble gases, dielectric-constant gas thermometry with helium, Doppler broadening of absorption lines and total radiation thermometry are the favourite methods.

Dr. Buck will describe the principles and actual uncertainties of these methods and evaluate their estimated contributions to a new value of the Boltzmann constant. After a new definition of the kelvin temperature can be measured with different but equivalent methods which are no longer based on a special substance but valid and reproducible, if necessary, within the whole universe.

Wolfgang Buck received his PhD for research on reactions of light nuclei at the University of Tuebingen (Germany) in 1976, where he subsequently joined as a member of academic staff. His interest then changed to the break-down of superconductivity in thin-film microbridges caused by the increased transport current. In particular, he has studied experimentally the flow of magnetic flux tubes and explained their behaviour by numerical simulations. In 1979 Dr. Buck joined the Physikalisch-Technische Bundesanstalt in Berlin and became a member of the ultra-low temperature group, which contributed substantial measurements to the Provisional Low Temperature Scale (PLTS-2000), which was adopted by the CIPM in 2000. His responsibility was the millikelvin and microkelvin cooling equipment and the scale carrying 3He melting curve thermometer. After the unification of Germany he was engaged in several problems of the integration of the members of the East-German NMI into PTB. Since 2002 he has been appointed as the head of the division "Temperature and Synchrotron Radiation" and as the Head of the Berlin Institute of PTB, as well. The most important among his many responsibilities are to push the German contribution to the redefinition of the kelvin and to promote the new low-energy synchrotron radiation source of PTB named Metrology Light Source, at present under construction.