ezra . steinberg
Sat, 14 Aug 2010 21:02:27 -0700
While almost certainly too much to hope for, redefining the kilogram in the way described below would also present the prefect opportunity to give 1 N.s^2/m a new name (and thus remove any prefix for the base definition of mass).
-- Ezra
----- Original Message -----
From: "Anthony O'conner" <barkatf...@ymail.com>
Sent: Saturday, August 14, 2010 4:37:16 PM
Subject: [USMA:48373] Electrical Standards article
Bill,
Check out USMA 47922 for some details on definition of the units to be proposed
to the CGPM in 2011.
I copied it below:
Anthony
----- Forwarded Message ----
From: "Anthony O'conner" <barkatf...@ymail.com>
To: usma@colostate.edu
Sent: Wed, July 7, 2010 4:15:13 PM
Subject: [USMA:47992] Nice BBC article that is all metric (and my show the path
to redefining the kilogram?)
I wonder 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 volt.
Here is why:
The coulomb is actually precisely defined. A coulomb is then equal to exactly
6.241 509 629 152 65 × 10 18 positive 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 Measures as: K {J-90} = 2 e / 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 K
J-90 . The CODATA 2006 value, on the other hand, is K J = (483 597.891±0.012) ×
10 9 Hz/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