Yes, folks have been working on a possible restructuring the definitions
that the SI's base units are based on.
At the annual meeting of the IEEE Standards Coordinating Committee 14 we
reviewed and commented on one such proposal. The matter is rather
complex and I expect to submit an article to Metric Today in the near
future on what is being planned.
No doubt, the kilogram artifact currently residing in France will be
replaced as a standard. Previous issues of Metric Today have discussed
two main possible means of doing this -- a definition based on
Avogadro's number and a definition based on Planck's constant. Possible
realizations for those two methods are, respectively, "counting" the
atoms in a sphere of Si and weighing a mass on a Watt balance. One of
those two likely will be the "winner", once the dust has settled.
As soon as I obtain the updates I have requested on this topic, I will
submit an article to Metric Today and, if it is accepted, I will alert
you via this forum. It would be premature to say much more at this time
since what I last saw and discussed may have changed considerably.
I do urge those who take part in this forum to consider becoming USMA
members if they have not already done so. This of course provides you
with a subscription to Metric Today.
Jim Frysinger
Chair, IEEE SCC 14
Vice Chair, IEEE/ASTM Joint Committee for Maintaining SI 10
Bill Potts wrote:
Simon:
There is work afoot to dissociate the kilogram from an artifact. I have
some information on that, but I'm not sure it's available for public
distribution yet.
However, Jim Frysinger can probably tell you what the approach is
without having to look it up.
Bill
------------------------------------------------------------------------
Bill Potts
WFP Consulting <http://wfpconsulting.com/>
Roseville, CA
http://metric1.org <http://metric1.org/> [SI Navigator]
------------------------------------------------------------------------
*From:* [email protected] [mailto:[email protected]]
*On Behalf Of *[email protected]
*Sent:* Monday, August 10, 2009 20:31
*To:* U.S. Metric Association
*Subject:* [USMA:45578] defining the kilogram from the joule
Why is the kilogram a base unit in SI instead of the joule? The
kilogram is the only base unit still defined from an artifact.
Since the artifact can change with time and the kilogram has been
know to do so, wouldn't it be better if a different unit were to
replace the kilogram as a base unit?
Energy is the most fundamental substance in nature. Energy has
always existed, even before the universe was created and will exist
long after the universe is gone. Energy is universal. Wouldn't it
be simpler to define one joule of energy from basic principles in
physics and define the kilogram from the joule in the relationship
that 1 kg = 1 J s**2/m**2?
Or, the kilogram can be defined from the newton, the meter and the
second, which are already accurately defined.
The ampere also should not be a base unit, the coulomb should be.
Since the ampere is defined from the newton and the newton is
defined from the kilogram, then the practical realization of the
ampere is affected when the kilogram changes.
From the Wikipedia article on the coulomb:
/In principle, the coulomb could be defined in terms of the charge
of an //electron/ <http://en.wikipedia.org/wiki/Electron>/ or
//elementary charge/
<http://en.wikipedia.org/wiki/Elementary_charge>/. Since the values
of the //Josephson/
<http://en.wikipedia.org/wiki/Josephson_constant>/ (CIPM (1988)
Recommendation 1, PV 56; 19) and //von Klitzing/
<http://en.wikipedia.org/wiki/Von_Klitzing_constant>/ (CIPM (1988),
Recommendation 2, PV 56; 20) constants have been given conventional
values (K_J ≡ 4.835 979 × 10^14 Hz/V and R_K ≡ 2.581 280 7 × 10^4
Ω), it is possible to combine these values to form an alternative
(not yet official) definition of the coulomb. A coulomb is then
equal to exactly 6.241 509 629 152 65 × 10^18 elementary charges.
Combined with the present definition of the //ampere/
<http://en.wikipedia.org/wiki/Ampere>/, this proposed definition
would make the //kilogram/ <http://en.wikipedia.org/wiki/Kilogram>/
a derived unit./
//
The ampere can then become a derived unit (1 A = 1 C/s) that is very
accurately defined since the coulomb and the second are accurately
defined. The newton would be defined as:
1 N = 1 J/m.s; if the joule is base unit defined from some principle
of physics. The kilogram would then be defined from the newton as 1
kg = 1 N s**2/m**2. Otherwise, the newton would have to be a base
unit and defined from the same rule that defines the ampere, but in
reverse.
Either way, this would eliminate the kilogram being tied to an artifact.
Simon
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
(F) 931.657.3108
