Has the kilogram changed over time?

[Robert Fogt]

http://www.foxnews.com/scitech/2010/10/29/kilogram-standard-invalid-nist/

[Mrs X]

It's been re-defined a couple of times too. Fascinating how easily doubt is cast on something you think is a fundamental. (At least for me, lol).

[JohnS]

Yes, the fundamental prototype in Paris and the various national prototypes are slowly changing relative to one another. Although it is VERY slow, metal atoms can evaporate from the surface (goes much faster at higher temperatures). The changes are of the order of micrograms in a kilogram, but that is a lot to a metrologist.

The US is not alone in the quest for a better kilogram. Several nations have worked on both the watt-balance approach, and a silicon sphere in which the number of atoms can be counted. In toto, the proposal also changes the mole, kelvin, and ampere to base all four on fundamental constants (I think Fox is wrong on the candela). I believe in the CGPM process, someone has to step forward and take the lead in making a proposal, but the real work has been a multi-national effort.

A draft or placeholder of the revisions to The SI Brochure has been released for comment. It states the proposed definitions, but the fundamental constants are only stated to limited accuracy (6 digits), followed by an X for the missing digits. A link to the draft was posted Sep. 29 on the USMA list server archive. The final values of the fundamental constants will be proposed by the committee that publishes the CODATA values (can't recall their formal name).

The draft fixes the value in roughly the same way as fixing the value of the meter by declaring the speed of light to be an exact constant. The meter is the length that "makes it so."

On a MUCH lighter note, the irony of a nation which is not officially metric taking the lead in redefining the metric system can't be dismissed. Of course the reality is that science and several industries ARE metric, the public at large is not so metric.

Edit: Links to the USMA list server, and draft SI Brochure for anyone interested:
http://www.mail-archive.com/usma@colostate.edu/msg37896.html
http://www.bipm.org/cc/CCU/Allowed/20/Watermark_SI_Brochure_Draft_Ch2_29Sep10.pdf

Edit2: This story may be clearer than the Fox story:
http://www.laboratoryequipment.com/News-revamping-the-international-measurement-system-102710.aspx?xmlmenuid=51