Redefining the Kilogram

January 26, 2011

A while ago, I posted a note about the kilogram’s weight-loss problem.   The kilogram is the only fundamental unit of the SI [Le Système International d’Unités] system of units that is defined by a physical object: the mass of a particular cylinder of platinum/iridium alloy, stored in a vault at the Bureau International des Poids et Mesures [BIPM] at Sèvres, outside of Paris.  As I mentioned in that earlier post, that cylinder appears to be losing weight, at least by comparison with copies of it that have been distributed to various national metrology labs.   Since there is no more authoritative source with which to compare it, no one is quite sure what is going on.  All of the other fundamental SI units have been re-defined in terms of physical processes.  For example, the meter, originally defined as 1/10,000,000 of the distance between the earth’s Equator and the North Pole, is now defined as the distance that light travels in 1/299,792,458 second.

The New Scientist site has an article on the ongoing effort to find a new definition for the kilogram.

In October, the General Conference on Weights and Measures in Paris is expected to begin the process of changing the definition of the kilogram to one based on fundamental constants like Avogadro’s constant, the number of atoms in a mole, and the Planck constant, which relates the energy of a photon or particle to its frequency. If everyone can agree on the technologies to do this, the redefinition process should be completed by 2015.

There are two principal contenders for the new definition.  One is based on a device called a Watt balance, which I mentioned in my earlier post.  This is essentially like a regular balance, except that the force on one side is electromagnetic, rather than being provided by gravity.  The idea is that the “master” kilogram could be weighed in the Watt balance; once that was done, and the balancing voltage and current are known, then an accurate comparison of any object could be made without reference to the physical cylinder.   Obviously, this would require a very accurate and stable method of measuring the current and voltage.

Another approach is outlined in a note [abstract] in this week’s edition of Physical Review Letters.  This involves using a new method to create a sphere of silicon-28 with a known number of atoms.

A team led by metrologist Peter Becker of the Federal Institute of Physical and Technical Affairs in Braunschweig, Germany, reveals a breakthrough in an attempt to measure the number of atoms in a silicon sphere, which has let them compute Avogadro’s constant to unprecedented accuracy.

Using their technique, the team has been able to determine the value of Avogadro’s constant (approximately 6.02×1023) to an accuracy of ±3.0×10-8. They feel that, if they can approximately double this accuracy, they will have the best candidate for a new definition of the kilogram, based on the mass of a certain number of silicon-28 atoms.

Getting to a new, agreed definition will of course involve a good deal of discussion, and perhaps some disagreements.  However, it’s probably useful to remember that this problem exists only because we have learned to measure the physical world to a degree of accuracy undreamed of when the metric system was first formulated in the 18th century.


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