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Degrees Kelvin

Date: 04/16/2002 at 22:31:02
From: Milan
Subject: Degrees Kelvin

Why is it considered to be wrong when you write, for example, 
0 degrees Kelvin? I have been told by my math teacher to write 
0 Kelvin instead, without the degrees. I find this quite awkward as I 
am used to writing the 'degrees'.  I was wondering if you could 
explain to me why.  

Thank you.


Date: 04/17/2002 at 09:06:28
From: Doctor Peterson
Subject: Re: Degrees Kelvin

Hi, Milan.

I can't say I understand why either, but this is the decree of the 
International Bureau of Weights and Measures, the organization that 
defines the SI metric system:

    How Many? A Dictionary of Units of Measurement - Russ Rowlett
    http://www.unc.edu/~rowlett/units/dictD.html 

    degree Kelvin
    an obsolete name for the kelvin. In the International System,
    temperatures on the absolute temperature scale are stated in
    kelvins, not in degrees Kelvin. 

For more detail, see:

    NIST Reference on Constants, Units, and Uncertainty
    http://physics.nist.gov/cuu/Units/kelvin.html 

    The definition of the unit of thermodynamic temperature was
    given in substance by the 10th CGPM (1954) which selected
    the triple point of water as the fundamental fixed point and
    assigned to it the temperature 273.16 K, so defining the
    unit. The 13th CGPM (1967) adopted the name kelvin (symbol K)
    instead of "degree Kelvin" (symbol K) and defined the unit
    of thermodynamic temperature as follows: 

    The kelvin, unit of thermodynamic temperature, is the
    fraction 1/273.16 of the thermodynamic temperature of the
    triple point of water. ...
    
    The unit of Celsius temperature is the degree Celsius,
    symbol C, which is by definition equal in magnitude to the
    kelvin. A difference or interval of temperature may be
    expressed in kelvins or in degrees Celsius (13th CGPM, 1967). 

I have never found an actual explanation of the reason for this 
change. My guess is that they want it to look like any other SI unit. 
The various kinds of degrees are labeled as "degrees" (the unit 
itself) followed by "Celsius" or whatever (identifying the system 
whose degree and 0 point are being used). Since the kelvin is the 
standard in SI, I suppose they feel there should be no need for such 
a two-level name, which suggests that there might actually be rivals 
to the SI!

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/ 
Associated Topics:
Middle School Temperature

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