Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: do not use calories/SI and nothing else



This is a repost of a post that I sent in yesterday morning. Apparently it
got lost whan the phys-l server temporarily quit working yesterday afternoon.

A few days ago Ludwik wrote:

Kilogram is now officially defined in terms of the mass of C-12. ....

As far as I know this is not the case. I think the kilogram is still (since
1901) the mass of a particular artifact (International Prototype kilogram)
kept at the BIPM at Sevres, France. (At least this seems to have been the
case as of the Aug. 1996 NCSL Workshop and Symposium on Metrology in Boulder,
CO whose list of abstracts is available at
http://ts.nist.gov/ts/htdocs/ncsl/abstract.htm.) Recently there have been
repeated calls within the international metrology community to change the
definition of the kilogram to a more modern definition. One proposal that I
have heard of is to define: 1 kg = N_A*m_12/0.012 where N_A is a conveniently
*defined* value for Avagadro's number and m_12 is the mass of a C-12 atom (in
its ground state). Traditionally, (at least for a few decades) the above
equation has been turned around and used as a definition of N_A in terms of
the traditional definition of the kilogram. As far as I know this above
proposal has not been officially adopted for lack of a sufficiently precise
agreed upon value for N_A. The best method for counting macroscopic numbers
of microscopic things is in terms of the measured lattice spacing of large
pure defect-free single crystals using crystallography techniques using a
combination of x-ray and optical interferometric schemes. The crystals of
choice for this purpose are those of Si. By careful mass spec measurements a
very precise ratio of the mass of a Si-28 atom to that of a C-12 atom can be
made. Thus such a crystallography measurement can yield a very accurate
value for N_A. As I recall the powers that be did not want to change the
definition of the kilogram until an agreed upon value of N_A (suitable for
using as a defined standard) was determined to better than 50 parts per
billion using the current definition of the kilogram. The major hangup here
was in getting a very accurate handle on the problem of the size and density
of lattice defects in single crystals of Si.

Maybe I'm wrong here and the kilogram definition has already been officially
changed. If so I just haven't kept up with the most recent changes. If
anyone on the list knows the actual *current* facts of the situation could
that person step in and explain them more fully? If the kilogram has been
changed then what is the official adopted defined value for N_A? The
accepted experimentally measured value for N_A since 1986 has been
6.0221367(36) x 10^23 mol^(-1).

David Bowman
dbowman@gtc.georgetown.ky.us