A new definition of the kilogram?
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
June 17th, 2007 at 12:28
Fixing the value of the kilogram?
Would it be possible to change the value of the kilogram so that it was exactly equal to the mass of 1 litre (1 cubic decimetre) of water at 3.98 deg. C (maximum density) and atmospheric pressure?
The standard kilogram was meant to have this value of mass, but an error in its manufacture meant it actually had the mass of 1.000028 cubic decimetres of water.
June 17th, 2007 at 19:48
In answer to the points made by George Carty:
It is important that we don’t change the value of the kilogram.
Although it was originally tied to the properties of water, advancements in measurement technology and physics generally showed that the density of water was subject to too many variables. It was a bit like the metre being bound to a quadrant of the Earth which subsequently turned out not to be the uniform geometric shape envisaged at the time.
The problem facing 21st century metrology is to establish a stable, reproducible reference for mass that does not invalidate existing data (i.e. mass measurements made with reference to the current prototype) along the same lines as the metre and its original prototype.
June 18th, 2007 at 11:18
Following on George’s comments - when the kilogram was first defined, people’s understanding of thermodynamics was significantly less than it is now – in particular I do not believe that the behaviour of saturated vapour pressure was fully understood in 1799 – Clapeyron who pioneered that work published his papers in the 1830’s.
If we were to use one litre of water at its maximum density as the definition of a kilogram, we have problem – it we weigh the water in vacuum, we will not have a vacuum for long – some water will evaporate, so we could weigh the water in air and take the air’s buoyancy into account. Again, either the water would evaporate or if we used fully saturated air, we would get condensation where we do not want it.
We could try putting the water into a sealed container, but making the container exactly one litre to within one part in 10^8 presents engineering problems that are probably insurmountable. We would furthermore need to ensure that the water in the sealed container was at a specified pressure – water is slightly compressible.
All told, I think that it can be seen that using water instead of a metallic solid presents many problems. The technique used for updating standards is to use the best possible technique of the day and as part of the change-over to calibrate the new standard so that it matches the old as closely as possible.
June 23rd, 2007 at 16:34
> better than 1 nm (which is approximately two wavelengths of light)
Since the wavelength of visible light is 400–700 nm, two wavelengths of light are about 1 µm, not 1 nm.