Talk:International System of Units/Archives/09/2011

Why base unit for amount of substance?
Quote from http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Science#Two_SI-units_for_amount_of_substance._WHY.3F

Couldn't the answer to that question be included in the article? Bo Jacoby (talk) 19:22, 19 July 2011 (UTC).
 * The SI article says: The seventh base unit, the mole, was added in 1971 by the 14th CGPM. But according to the ideal gas law, PV/T is already a measure of amount of substance. The corresponding SI-unit is joule per kelvin. The conversion factor is called the gas constant. One mole is simply equal to 8.314 joule per kelvin. Can anyone tell me why it was standardized?


 * No, the gas constant is 8.314472(15) J·K−1·mol−1. J IM ptalk·cont 21:18, 19 July 2011 (UTC)

Thanks for the extra precision. Do you know the answer to my question? Bo Jacoby (talk) 21:28, 19 July 2011 (UTC).


 * I'm not sure what you mean by "standardised". The gas constant is a measured quantity (unlike the speed of light which defines the metre in terms of the second). J IM ptalk·cont 23:58, 19 July 2011 (UTC)

I am talking about the 14th CGPM making the mole the seventh base unit of the International System of Units. Bo Jacoby (talk) 11:13, 20 July 2011 (UTC).
 * The gas constant is not known with perfect precision. Making the mole a base unit means it does not have to be redefined as measurements become more exact. Jonathunder (talk) 13:15, 20 July 2011 (UTC)

That argument didn't make the astronomical unit an SI base unit. Bo Jacoby (talk) 15:44, 20 July 2011 (UTC).
 * Earth's orbit is not any kind of fundamental constant. In any case, this is not a forum to argue if it should be a base unit; there are other places for that. Jonathunder (talk) 17:03, 20 July 2011 (UTC)


 * This disagreement seems to be about the difference between a base unit and a constant. The ideal gas law may be written n = PV/RT where n is measured in the base unit moles. The units of all measured quantities may be reduced to a relationship between base units. The 14th CGPM just recognized that amount of substance needed to be added to complete the list of base units. Physical constants can all be measured using only the seven base units. The definition of the amount of a single amount of an SI base unit (i.e., 1 metre, 1 second, 1 kilogram, etc.) is, however, somewhat arbitrary although efforts have been made to base these amounts on physical and mathematical constants, e.g., speed of light, triple point of water, etc. Dger (talk) 17:40, 20 July 2011 (UTC)

Thank you. Bo Jacoby (talk) 19:24, 20 July 2011 (UTC).
 * Neither earth's orbit nor the mole are fundamental constants.
 * The ideal gas law may be written n = PV/T where n is measured in the SI unit J&middot;K&minus;1.
 * The reason why a seventh base unit was added, should be reported in the article, and if no such reason exist, then that fact should be reported in the article. I do not know the reason why.
 * The mole is not a fundamental constant it is a base unit for the "construction" of other units of measure, mostly chemical. The n in your equation is not the same n as in the one I wrote. My n is amount of substance in moles, a base unit. Yours is units of joules/kelvins, a completely different beast. Someone may want to add an explanation for why to include the mole but not me. It needs a chemist or physicist. We might than have to write a reason for every other base unit as well. Most seem very obvious why they are included. Dger (talk) 01:08, 21 July 2011 (UTC)

Thank you. The two n 's are like measuring some length in centimeter and in inch, and inch is a completely different beast than centimeter, even if the length is the same. Including the mole as a base unit seems conspicuously unnecessary and inappropriate, and an explanation is most urgently needed in this case. Bo Jacoby (talk) 06:01, 21 July 2011 (UTC).
 * Sorry, no, the two ns are not the same thing. They are different quantities. The J/K reduced to base units is (kg·m2)/(s2·K), which is NOT the same as a mole nor is it like the difference between a centimetre and an inch. I can assure you that indeed the mole is a necessary base unit. It would not have been added if it wasn't. Look up the mole in Wikipedia. Dger (talk) 14:22, 21 July 2011 (UTC)

Our fundamental unit article is more of an essay than it should be, but it's a well written essay and it may be of interest. Jonathunder (talk) 14:51, 21 July 2011 (UTC)

(ec)Thank you Dger. I did look up the mole in Wikipedia, and yes, please explain to me that indeed the mole is a necessary base unit. I too was confident that it would not have been added if it wasn't, but now I doubt it. The mathematical fact is that if n=PV/TR, where R is a constant, is the amount of substance, measured in mole, of an ideal gas, then nR=PV/T is the amount of substance measured in J&middot;K&minus;1. According to http://physics.nist.gov/Pubs/SP330/sp330sl.pdf page 33:
 * The 14th CGPM, considering the advice (...) concerning the need to define a unit of amount of substance, decides 1. The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; (...)

It follows that the 14th CGPM was not adviced that the J&middot;K&minus;1 is an SI unit for amount of substance. Having been so adviced the decision would not have made. Bo Jacoby (talk) 15:35, 21 July 2011 (UTC).


 * The history of the metric system article, particularly |this section, may have useful information. Jonathunder (talk) 20:52, 21 July 2011 (UTC)


 * Just to wrap this issue up, there is some disputes whether the mole should be included. These can be found at Mole (unit). My feeling is that this article on SI units is the place to state what are the currently accepted base units. Personally, I would like to see the radian and Dger (talk) 17:05, 24 July 2011 (UTC)

I forwarded the following email to the BIPM.
 * Excellencies of the Bureau International des Poids et Mesures.
 * I have a question regarding why the mole was standardized as a base unit in the SI. According to the ideal gas law, PV/T is a measure of amount of substance. The SI-unit is joule per kelvin. The conversion factor between mole and joule per kelvin is the gas constant R. There is no need for having two SI units for amount of substance. Can anyone please tell me why the mole was standardized?
 * Yours truly Bo Jacoby

Bo Jacoby (talk) 16:49, 9 August 2011 (UTC).

Hi! The answer to the original question is that "amount of substance" is (nowadays) the official scientific name for the quantity that in common language we call "count". The "mole" is a very big counting unit, suitable for counting atoms (etc.). More familiar counting units are: "pair", "dozen", "score", "gross", etc. A counting unit is not a pure number. For example, think about the sentence. "In the test, he came in position "dozen". Whether you need to formalise counting in this way is another issue. Presumably the relevant international bodies though that it would be helpful, particularly in chemistry.RGForbes (talk) 12:07, 1 September 2011 (UTC)

Include Unicode characters for SI Units?
I'm sure someone would have already proposed this... But I cannot find the discussion:

It would be useful for someone that needed to know these SI units in Unicode.

NevilleDNZ (talk) 15:22, 3 September 2011 (UTC)
 * I like the idea, but perhaps this should be moved to a sub-article linked to from the SI units article? In order to further improve maintainability and cross-platform compatibility I suggest to HTML encode the Unicode characters, as not all systems have character sets set up correctly, and not all browsers and operating systems support Unicode. This can led to the display of garbage instead of the Unicode characters. It is easy to avoid this by replacing the immediate use of Unicode characters as "㎏" by instead. On systems supporting Unicode, there will be no difference in display, however, looking at the source code, it is possible to find out the actual code points used, and on systems not (or not properly) supporting Unicode you will see something like &amp;#x338F; instead of useless "?" or plain garbage. To illustrate this, I changed this in the first of your tables and also added a new column giving the actual codepoint, so people can enter these characters without relying on copy&paste. --Matthiaspaul (talk) 16:19, 3 September 2011 (UTC)


 * I would prefer to see this as a separate article as it could attract a large discussion subsection - in real life I work in IT and my philiosphy is not to rely on users having the most modern equipment - in particular one often finds that equipment is tuned to only handle a subset of Unicode (MES-1, MES-2 or MES-3 for example). A discussion on the incompatibilty of special Unicode characters with these subsets might well overwhelm the current article when, in reality, it is only a footnote. Martinvl (talk) 16:57, 3 September 2011 (UTC)


 * When we start talking about Unicode we're way off on a tangent. This either deserves its own article or belongs on the Unicode article. J IM ptalk·cont 17:58, 3 September 2011 (UTC)


 * So, it seems we all agree on this. What about the original poster? --Matthiaspaul (talk) 07:59, 6 September 2011 (UTC)

An unlimited number of derived units
The article currently claims that there are an unlimited number of derived units formed from multiplication and division of the seven base units. Yes, {ma·kgb·sc·Ad·Ke·cdf·molg} where a, b, c, d, e, f and g ∈ ℕ is an infinite set but it's populated by unity, a finite number of units which are used plus an infinite number of entities which never have been used. Can we rightly call such entities units? m12319·kg−3455·s3127·A10023787·K−267382·cd773636·mol−76527635 has no use now & will probably never have a use. Is it a derived unit? J IM ptalk·cont 01:13, 6 September 2011 (UTC)
 * In the infinite future who knows that such a unit won't be needed? Dger (talk) 02:43, 6 September 2011 (UTC)
 * There is nothing in the rules that prevents such a unit of measure existing. Moreover, certain constants of proportionality have non-integer powers. Martinvl (talk) 07:35, 6 September 2011 (UTC)


 * I think that was my text, paraphrased from the SI Brochure 8 "the number of derived units is without limit..." Adamtester (talk) 07:46, 6 September 2011 (UTC)
 * I have reinstated the original text, but used the wording "unlimited in number" which I believe to be an accurate paraphrase of the SI brochure. I have also included a reference to the SI brochure to this wording. Martinvl (talk) 13:17, 6 September 2011 (UTC)

Naming units by their symbols
I'm french and I'm often in India and there, they spell units: "key-jee" for kg, "em-em" for mm, etc..

It makes me bristle, but prior to rebel, does anyone know if this is an "Indianism" or is it common to English-speaking countries? — Preceding unsigned comment added by 121.245.20.27 (talk) 05:54, 11 September 2011 (UTC)
 * I believe that it is an Indianism - I have certainly never seen it, but then I have never been to India. It is certainly not part of the official defintion of SI. Martinvl (talk) 07:33, 11 September 2011 (UTC)
 * It's not the norm in English, though, you might hear some units named this way sometimes. J IM ptalk·cont 00:30, 12 September 2011 (UTC)