Date: 04/17/2002 at 09:06:28From: D

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/