Iwonder how many USMA members are aware of what is going on with the kilogram
and why it is important to find a new definition quickly.Presently there are 15
units that are defined using the kilogram (8 aren't) :The newton, joule,
pascal,
watt, volt, ampere, coulomb, farad, tesla, henry, weber, ohm, siemens, katal
and
mole are defined from the kilogram either directly or indirectly. Only the
metre, second, kelvin, candela, radian, steradian, lumen and lux aren't. Did I
miss any? Any change to the kilogram changes all 15 of these units. That is
not
good.
Presently, there is some work being done to fix this. I expect some major
rearrangement of the units. For example I don't see the kilogram or the ampere
remaining as base units. Instead, expect them to be replaced by the coulomb
and
the newton.
Here is why:
The coulomb is actually precisely defined. A coulomb is then equal to exactly
6.241 509 629 152 65 × 1018positive elementary charges. It is expected that at
the next CGPM a proposal will be made to define the coulomb this way and the
coulomb can replace the ampere as a base unit. Since an ampere is defined as
one coulomb per second and both the coulomb and second are precisely defined,
the ampere will be precisely defined.
The Watt-Balance Experiment
Since the watt is equal to both the ampere volt and the newton metre per
second,
and the ampere, metre and second are precisely defines, all that needs be done
is to precisely define either the volt or the newton.
Experiments are being done to precisely define the volt using the Josephson
Junction definition. Since 1990 the volt has been maintained internationally
for
practical measurement using the Josephson effect, where a conventional value
is
used for the Josephson constant, fixed by the 18th General Conference on
Weights and Measuresas:
K{J-90} = 2e/h = 0.4835979 GHz/µV.
International Committee for Weights and Measures, or CIPM, recommended that
this
be considered the exact "conventional" value of the constant, denoted KJ-90.
The CODATA2006 value, on the other hand, is KJ= (483 597.891±0.012) × 109Hz/V
[2]
This in effect defines the volt relative to the second, a precisely defined SI
unit.
Once the value of K is precisely resolved to the satisfaction of the
researchers, it can be used define the volt and make the volt a base unit.
Once
this is done it will be easy to define the newton as being one volt coulomb per
metre. With the three units precisely defined, the newton will become a
precisely defined unit.
Then the kilogram can be defined from the newton. 1 kg will then be defined as
1 N.s^2/m.
Thus the problem with using a changeable artifact to define the kilogram, the
kilogram and all SI units will be defined from precise natural phenomenon.
Does anyone who reads this have any heads-up on what is happening on this
subject?
[USMA:47992] Nice BBC article that is all metric (and my show the path to
redefining the kilogram?)
ezra . steinberg
Mon, 28 Jun 2010 21:07:51 -0700
This science article from the BBC uses metric only (which is nice to see):
http://news.bbc.co.uk/2/hi/science/nature/8767763.stm
It also makes me wonder if being able to measure gravity this finely could be
part of a redefinition of the kilogram to replace the current physical
artifact.
Or am I totally off base?
-- Ezra
