[USMA:24400] RE: We sponsor a Political Action Committee

[Bill Potts]

Carl Sorenson wrote:
>I don't really have sources for this, but this is what I assume. 
>Centigrade
>was defined in terms of the freezing and boiling points of water.  Celsius
>is (at least currently) defined in terms of Kelvin (which uses the triple
>point).  Thus, Celsius is tied to SI and centigrade is not, if this is
>correct.  In practical terms the difference negligible, but Celsius is the
>term to use.

You have it backwards (or maybe a little bit sideways), Carl.

Centigrade was an naming anomaly, based on the division of an arbitrary range into hundredths. The other scale, Fahrenheit, was, like so many other units, based on the name of a scientist. As the "Centigrade" scale was developed by Anders Celsius, his name was chosen (from three in common use -- Centigrade, Celsius and Centesimal) as the definitive one to clarify which particular degree was being expressed. Because it isn't itself a unit (but a qualifier of another unit, the degree), it retains the initial capital letter.

The kelvin (small k and named after Lord Kelvin) is a renaming of "degrees absolute" (for a while, called degrees Kelvin), which used the degree Celsius interval, but a zero point based on absolute zero (no molecular movement). The kelvin is most certainly not based on the triple point, though its relative value (between n k and n+1 k) is equal to the that of the degree Celsius.

Bill Potts, CMS
Roseville, CA
http://metric1.org [SI Navigator]