Physical Society Colloquium
Mass Metrology:
its Pitfalls and the Redefinition of
the Kilogram
George Chapman
Institute for National Measurement Standards
National
Research Council
In the popular mind and, indeed, that of many scientists, the measurement of
the mass of an object would seem to be a simple task, particularly with the
ready availability of modern comparators operated by microprocessors.
However, it is a truism that, with higher resolution and accuracy, effects
that were once negligible become important. Electromagnetic and tidal
effects are just two of the modern plagues of mass metrology, to be added to
the standing list of buoyancy and surface effects that have always made the
mass metrologist's life interesting.
Mass is the last of the base units of the Système International to be
embodied in a physical artifact a fact that should keep physicists awake at
night. It is also the only base unit that is inferred from the measurement of
something else. Efforts to redefine the kilogram in terms of atomic or
physical constants are ongoing and have evolved into three principle paths
based on electromagnetic force, Avogadro's number and the absolute masses of
certain isotopes. These will be reviewed and I will once again look into the
rather cloudy crystal ball which purports to provide a window into the future
of mass, its traceability and its reproducibility. Be warned that, as an
optimist, my predictive track record is not a particularly good one.
Friday, November 26th 2004, 13:30
Ernest Rutherford Physics Building, R.E. Bell Conference Room (room 103)
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