Bill,
This has been discussed, at least in part, in Metric Today. Jim On 2010-08-14 1446, Bill Hooper wrote:
There's an interesting article in The August 2010 issue of Physics Today, a magazine of the American Institute of Physics. It is about Electrical Standards and how they are related to mechanical standards, specifically how the SI units of electrical quantities are related to the SI mechanical units. Apparently there are possible ways of defining the SI units of voltage (the volt, V) and of electrical resistance (the ohm, Ω) in terms of fundamental constants of nature. That would lead to defining the SI unit of electric current, the Ampere, through Ohm's law (current equals voltage divided by resistance, therefore, the size of ampere (A) would be defined by 1 A = (1 V)/(1 Ω), or just A=V/Ω. There are apparently studies going on that would support changing definitions of some mechanical units as a result of these ideas. The article mentions (although does not explain) that the use of the Watt Balance, currently under consideration for use in the SI definition of the kilogram, could be used to replace the Standard Kilogram artifact (here called "Le Grand K"). Such a definition of the kilogram has been informally called "the electronic kilogram". If that can be done, then some of the mechanical units, including the kilogram, can be defined through Ampere's law which relates a force produced by an electric current. Currently, the ampere of current is defined in terms of the newton of force which in turn is defined in terms of the kilogram of mass. If the ampere is define from purely electrical relations, then Ampere's law could be used in reverse, to define the newton of force and thence the kilogram of mass. The article is relatively brief and does not try to explain in detail all the relationships necessary to define the newton and the kilogram this way. Although I found some of the explanation to be lacking important details, it is likely that those in the field are familiar with them. However, it does make for heavy reading, even if the article is rather short (two pages). Of course, that lack of detail is the very reason why the article can be so short. I still can't understand how the Watt Balance can be used to define the kilogram but I now can at least understand why it should be possible. Bill Hooper 72 kg body mass* Fernandina Beach, Florida, USA * The kilogram used here to measure my body mass is defined in terms of Le Grand K, that chunk of metal stored in a museum or something someplace near Paris, not in terms of the Watt Balance. However the difference, I believe, would be of the order of 1 part per trillion or so.
-- James R. Frysinger 632 Stony Point Mountain Road Doyle, TN 38559-3030 (C) 931.212.0267 (H) 931.657.3107 (F) 931.657.3108
