Australia helps to redefine the kilogram

Small Business Minister Dr Craig Emerson today handed over two unique spheres that may redefine the ‘kilogram’ to the world.

All of the basic SI units are currently defined in terms of atomic quantities except for the kilogram, which is still defined as the mass of a particular artefact. The kilogram, the unit of mass in the International System of Units (SI), is presently defined as the mass of a platinum-iridium cylinder sitting in a vault in France. There is evidence that the mass of the 119-year-old cylinder may have slowly changed over time in a small but measurable way. These not only affect the accuracy of the measurement of mass but as the definition of the kilogram links directly to measurements for energy, electrical and chemical quantities, these, too, are affected.

There is an obvious resolution of this unsatisfying situation — define the unit of mass in terms of the mass of an atom. All atoms of a given isotope of a given element have the same mass, so there is no danger of losing or damaging the defining quantity, as there is with the kilogram artefact.

Australia’s National Measurement Institute (NMI) is part of an international collaboration that is measuring the number of silicon atoms in nearly perfect crystals whose mass is close to 1 kg. The crystals were cut and polished into extremely round and smooth spheres by techniques developed in CSIRO's Centre for Precision Optics for manufacturing precise optical components. The small errors in the shape of each sphere are measured by very sensitive mechanical instruments and the mean diameter is found by measuring many diameters with a laser interferometer.

Other laboratories will determine the isotopic composition of the crystals and the mean spacing between the silicon atoms. From all this information the number of atoms in each sphere will be calculated. The sphere will be then weighed against a kilogram standard to establish the mass of one silicon atom. Every step in this process challenges the limits of present technology; for instance, the mean diameter of the sphere must ultimately be measured to about the diameter of an atom.

For more information, see www.measurement.gov.au.