A new method of defining the kilogram is being sought by various teams of scientists around the world. However, it may be some years before a decision emerges. (NB – this will obviously not alter the actual size of the kilogram). This article, contributed by Martin Vlietstra, will be of interest to the more technically minded.
The kilogram is an anomaly in the world of physical constants – its current definition relies on a particular artefact or object – the prototype kilogram that is held by the BIPM on behalf of its “shareholders”, its subscriber governments. Every other physical constant is defined in terms of one or other physical phenomena that can, in principle, be measured in any laboratory in the world. Ever since the retirement of the prototype metre in 1960, scientists have been looking for a means of defining the kilogram by means of a scientific experiment and yet maintaining the accuracy that can be obtained using the prototype kilogram
One of the projects to redefine the kilogram is to define it in terms of a sphere of silicon. Such spheres are currently being produced in the laboratories of the Australian Council for Scientific and Industrial Research (ACSIR) – See http://www.theage.com.au/news/national/making-an-exact-difference/2007/06/14/1181414466901.html.
Once the sphere has been manufactured, there are a number of problems associated with defining the kilogram. Firstly, the diameter of the sphere must known to an accuracy of better that one part in 10^8. If the sphere has a mass of exactly one kilogram, its radius will be approximately 93.58 mm, so its diameter needs to be known to better than 1 nm (which is approximately two wavelengths of light). Details of some of the scientific techniques used and the participating laboratories (Australian, Belgian, British and German [in alphabetic order]) can be found at http://www.npl.co.uk/mass/avogadro.html.
In addition to measuring the diameter, the scientists concerned will need to identify which is the more practical – to define the kilogram in terms a specific number of silicon atoms or to define it in terms of the mass of a sphere of specified radius. Part of the experiments currently under way is to decide which of the two techniques give the better results.
This is not the only experiment that is being developed to redefine the kilogram; another is the Watt Balance which is being carried out by the BIPM. (See http://www.bipm.org/en/scientific/elec/watt_balance/ ).
Who will decide which experiment is the better? This will ultimately be decided by the CGPM on the advice of the CIPM and is likely to be some years off.
CGPM = Conférence Générale des Poids et Mesures / General Conference on Weights and Measures, a body consisting of representative of the governments that have subscribed to the Convention of the Metre.
CIPM = Comité International des Poids et Mesures /International Committee of Weights and Measures, a body of 18 eminent scientists elected by the CGPM.