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.

5 thoughts on “A new definition of the kilogram?”

  1. Crikey, what a muddle the world seems to want to be in! Absolute definitions for the length of a meter, a kilogram, and therefore area and volume, seem to be an eternal encumbrance.

    It all used to feel so simple to me. Anita was a subdivision as a distance from the North Pole to the equator. A kilogram is the weight of water corresponding to 10 cm³. Although, of course, this is somewhat temperature dependent. But as far as every day use is concerned, the North Pole to equator, and 10 cm³ measurements cover most of the bases for distance and weight and volume et cetera. Unless you are working at atomic scale. But that doesn’t concern the vast majority of us.

    Not to return to the topic at the beginning, it must be disappointing to some people that a sphere of silicon doesn’t come out at some nice convenient and round figure!

    But carry on, scientists the world over, I’m sure you’ll come up with some definitive way of judging all that which was started from most humble beginnings. That humble, baseline way of measuring things will do for most of us (-;)~


  2. Bob Phillips:

    It may sometimes seem strange to most of us that these absolute definitions are required. But I’d rather have a metre with an absolute definition based on physics and determined by scientists than a yard cast as an iron rod and put on show somewhere like Trafalgar Square or Greenwich where it will expand and contract with the weather.


  3. The ångström was orginally developed by Anders Ångström because he was measuring the wavelength of particular lines in the spectrum with a greater precision than was possbile by taking an absolute measure from the metre des archives. He took one particular line, measured it to the best accuracy he could using centimetres and then defined what became known as the ångström such that 10^8 ångströms were one centimetre. When the metre was redefined in 1960, the new definition was Ångström’s definition in reverse.


  4. It’s too bad that Anders Ångström didn’t have the vision to set his unit to 10^-9 m (10^-7 cm) so that like the micron being now equal to the micrometre, the Ångström would have been exactly the same as the nanometre and the numbers from either would be the same. Even though the nanometre is the standard today, Ångström and its different numbers are still mentioned. The same is also true between the millibar and the hectopascal. Would have been better if the millibar was equal to the kilopascal.


  5. Really, this is just the progress of science. As Bob Phillips says, all these base units had humble origins, which are still good enough for the vast majority of us. As science advances, greater accuracy is required for laboratory process and the need for more-accurate definitions emerges.


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