Thanks Jim for the heads-up! It seems though that there are going to be some major changes to SI come the 24-th CGPM in 2011. The ampere may be removed from being a base unit and replaced by the coulomb, where the coulomb will be defined as exactly 6.241 509 629 152 65 × 1018 elementary charges. The ampere will then be a derived unit from the coulomb and the second. Since the coulomb and the second will then be accurately defined, so will be the ampere.
The watt balance method would not define the kilogram directly. It would accurately relate mechanical power (watts) to electrical power (watts). Electrical the power equals voltage times current. Mechanical power equals force times velocity. In other words EI = FV The unit of force are the newton, the units of velocity are the meter per second, the unit of voltage is the volt and the unit of current is the ampere. This relationship allows for the use of the already accurately defined units of ampere (via the coulomb and the second), second and metre. The newton or the volt would need to be independently defined. The volt can be defined via the Josephson junction, or the newton can be defined via the inverse of the definition presently used to define the ampere. That is Ampère's force law applied in reverse. Which ever one becomes the easiest to realize would most likely become the base unit. The kilogram then would have to lose its status as a base unit and become a derived unit. It would be derived from the newton, the meter and the second. Since F = ma, it stands to reason that m=F/a and thus 1 kg would be defined as 1 N s**2/m**2. The result is that both the ampere and the kilogram will not and can not remain as base units. The coulomb will replace the ampere and either the volt or the newton will have replace the kilogram. I find this all very exciting. Simon PS. >From the Wikipedia article on the ampere: http://en.wikipedia.org/wiki/Ampere Realisation The ampere is most accurately realized using a watt balance, but is in practice maintained via Ohm's Law from the units of electromotive force and resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the Josephson junction and the quantum Hall effect, respectively.[10] At present, techniques to establish the realization of an ampere have a relative uncertainty of approximately a few parts in 107, and involve realizations of the watt, the ohm and the volt.[11] [edit] Proposed future definition Rather than a definition in terms of the force between two current-carrying wires, it has been proposed to define the ampere in terms of the rate of flow of elementary charges.[12] Since a coulomb is approximately equal to 6.24150948×1018 elementary charges, one ampere is approximately equivalent to 6.24150948×1018 elementary charges, such as electrons, moving past a boundary in one second. The proposed change would define 1 A as being the current in the direction of flow of a particular number of elementary charges per second. In 2005, the International Committee for Weights and Measures (CIPM) agreed to study the proposed change, and, depending on the outcome of experiments over the next few years, to formally propose the change at the 24th General Conference on Weights and Measures (CGPM) in 2011.[13] -------------------------------------------------- From: "James R. Frysinger" <[email protected]> Sent: Tuesday, 2009-08-11 10:23 To: "U.S. Metric Association" <[email protected]> Subject: [USMA:45581] RE: defining the kilogram from the joule > > 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 > >
