Pat, When I wrote "mixed units" I meant a mix of SI units with units which are not-SI units of multiples of SI units.
Precisely speaking, there is only one SI unit, the coherent SI unit, for any given quantity; with "multiples" (including "submultiples") of SI units formed by SI prefixes. However, as I noted previously, the 2008 editions of the BIPM Brochure and NIST SP 330 now recognize "multiples" of the limited set of coherent SI units as "units" as well, allowing a very large number of multiples (e.g. nm, mm, cm, km, etc. etc.) to be called "SI units." I prefer the clear distinction of "unit" to mean "coherent unit" which does not include "multiples" which are, however, useful for scaling numerical values up or down so that precision is not exaggerated. All of this pertains to *units* addressed by SI. The pound is outside SI, whether a currency, a mass, or a force; etc. for the large number of other "units" outside SI. e.g. A "unit" of dosage in pharmacy; a unit of housing, etc. The historical term "measures" means the *quantities* length, area, and volume in modern terminology. The historical term "weight" in the phrase "weights and measures" means only mass in modern terminology. Weight defined *only* as a force by the CGPM is a relatively recent development. I recommend that these distinctions be taught in school, but I do not favor punishments for abuses in common parlance. Gene. ---- Original message ---- >Date: Tue, 3 Feb 2009 10:24:46 +1100 >From: Pat Naughtin <[email protected]> >Subject: [USMA:42845] Re: Only "System of Units" (SI) >To: "U.S. Metric Association" <[email protected]> > > On 2009/02/03, at 4:22 AM, <[email protected]> > <[email protected]> wrote: > > A mix of units can be stated more simply as "mixed > units" or more precisely as "A mix of SI and > not-SI units" rather than > "use of multiple systems" as you propose, which > implies that more than one "system" exists. > > Gene. > > Dear Gene, > Just a slight quibble with your post that I have > been having trouble with for a while. > When is something a unit and when is it not? I am > referring to your use of 'mixed units' and 'not-SI > units'. > ... > Could you or anyone else tell me how you define what > a 'unit' is please? > Cheers, > > Pat Naughtin > PO Box 305 Belmont 3216, > Geelong, Australia > Phone: 61 3 5241 2008
