[Phys-L] Re: Animated Math - comments

["James R. Frysinger" <frysingerj@COFC.EDU>]

On Monday 04 July 2005 17:37, Bernard Cleyet wrote:
> "The SI derived temperature scale is the Celsius scale now, no longer
> called the Centigrade scale. The symbol is °C and the space comes between
> the number
> and the raised circle, not between the raised circle and the number. A
> raised
> circle immediately after the number (especially with a "C" following) would
> indicate a plane angle value given in degrees."
>
>
> Yes. Furthermore, I was taught that a number followed by a space and
> degrees C. is a temperature while a number followed by a space and C.
> deg. is a temp. difference or change.
> bc

        If I understand you correctly, Bernard, you are saying that you were taught
to use
                5 °C to indicate a temperature of 5 degrees Celsius and
                5 C° to indicate a temperature change of 5 degrees Celsius.
I have seen a textbook somewhere in recent history where the author created or
used that format. It is, however, not proper SI to do so. In the SI there is
no symbol C° and it represents nothing (unless one can imagine coulomb
degrees [of arc]). The SI is a creation of an international committee
structure that arises from an international treaty. It is not "up" for
adaptation by users except as its rules provide. It is defined and "owned" by
that committee structure --- or, if you will, by the many, many nations that
signed the treaty.

        Anyone who would propose to make this part of the SI can contact the
Consultative Committee on Units (CCU) to propose it, perhaps by going through
IUPAP. I think that the odds of that making it all the way to CIPM would be
about as good as a proposal to use
                5 Pa to indicate a pressure of 5 pascals and
                5 aP to indicate a pressure change of 5 pascals.
But that's just a guess on my part.

        The SI is a very concise set of units, prefixes, and rules. The tradeoff one
must make for such a tersely defined system is strict adherence to the rules.
That upsets lay people because lay people have always been cavalier with
units. It also upsets scientists who delight in forming their own "local
definitions", which perhaps plays to their creative spirits. Alas, both
instincts detract from the universal comprehension that the SI brings.

        One therefore must vent creativity on definitions of quantities and leave the
small set of units (and their symbols) unmolested. Good bye to Vac and Vdc,
folks! It's the voltage that changes, not the volt. There is only one volt.

        Likewise, there is only one degree Celsius. The SI brochure (see below) says
in Table 3, that °C = K. It also says, in note d, "This unit may be used in
combination with SI prefixes, e.g. millidegree Celsius, m°C." Now that blows
a lot of people's minds! Thus, the triple point of water occurs at a
temperature of 10 m°C.

        Thus the degree Celsius is a special name for the kelvin, one that is used on
a derived temperature scale. Conveniently or confusingly, that is called the
Celsius temperature scale and the values on that scale are stated in terms of
the degree Celsius, just as the values on the thermodynamic temperature scale
(nee Kelvin scale) are given in kelvins. (By the way, the degree Kelvin is no
more; now it is just the kelvin.)

        Another characteristic that folks have, be it lay or professional, is a
propensity to follow the examples of others instead of checking the source
documentation. That perpetuates a lot of bad information. For example, I was
amazed when I read Milliken's original papers and found out what he actually
concluded, in contrast to what what the textbooks all said that he had
concluded. Fortunately textbook writers today are more inclined to go to the
primary sources rather than "averaging" what existing textbooks say.

        Below are some links to references you might find handy for your reading
pleasure:
The defining document, commonly referred to as the "SI brochure", for the SI
in its authorized English translation:
   http://www.bipm.org/utils/en/pdf/si-brochure.pdf
The online version of the above:
   http://www.bipm.org/en/publications/brochure/online.html
The authorized U.S. adaptation of the SI, approved by the CIPM; this is what
authorizes -er endings instead of -re endings and L instead of l for the
symbol for the liter in the U.S.:
   http://physics.nist.gov/Pubs/SP330/sp330.html
A very handy guide that provides a bit more verbosity to clarify issues on how
one should use the SI; it is aimed at federal agencies but it is nicely
done!:
   http://physics.nist.gov/Pubs/SP811/sp811.html

Jim

--
James R. Frysinger
Lifetime Certified Advanced Metrication Specialist
Senior Member, IEEE

http://www.cofc.edu/~frysingj
frysingerj@cofc.edu
j.frysinger@ieee.org

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