Origins of Celsius and Fahrenheit

Date: 03/31/2002 at 00:52:02
From: Maranda Martin
Subject: Celsius and Fahrenheit

Dr. Math,

Who invented Celsius and Fahrenheit?

Date: 03/31/2002 at 08:35:20
From: Doctor Sarah
Subject: Re: Celsius and Fahrenheit

Hi Maranda - thanks for writing to Dr. Math.

Russ Rowlett's _How Many? A Dictionary of Units of Measurement_ is a 
good site for looking up this kind of information. Select D for degree 
Celsius and degree Fahrenheit. Here's what he says:

degree Celsius
a metric unit of temperature. The Celsius temperature scale is named 
for the Swedish astronomer and physicist Anders Celsius (1701-1744), 
who used a similar scale. The freezing point of water (at one 
atmosphere of pressure) was originally defined to be 0 degrees 
Celsius, while the boiling point is 100 degrees Celsius. Thus the 
Celsius degree is 1/100 of the difference between these two 
temperatures. (The scale actually used by Celsius was inverted, so 
that 0 degrees Celsius was the boiling point of water and 100 degrees 
Celsius the freezing point.) In the SI system, the Celsius scale is 
defined so that the temperature of the triple point of water (the 
temperature at which water can exist simultaneously in the gaseous, 
liquid, and solid states) is exactly 0.01 degrees Celsius, and the 
size of the degree is 1/273.16 of the difference between this 
temperature and absolute zero (the temperature at which all molecular 
motion ceases). For practical purposes this is equivalent to the 
original definition. 

degree Fahrenheit
a traditional unit of temperature still used customarily in the United 
States. The unit was defined by the German physicist Daniel G. 
Fahrenheit (1686-1736), who also invented the mercury thermometer. 
Fahrenheit set 0 degrees at the coldest temperature he could 
conveniently achieve using an ice and salt mixture, and he intended to 
set 100 degrees at the temperature of the human body. (He was off a 
little there; normal temperature for humans is between 98 degrees 
Fahrenheit and 99 degrees Fahrenheit.) On this scale, the freezing 
point of water (at normal sea level atmospheric pressure) turned out 
to be about 32 degrees Fahrenheit and the boiling point about 212 
degrees Fahrenheit. Eventually the scale was precisely defined by 
these two temperatures. 1 degree Fahrenheit equals 5/9 degrees 
Celsius, but in converting between scales we have to be careful to 
adjust the zero points as well. To convert a temperature in degrees 
Fahrenheit to the Celsius scale, we must first subtract 32 degrees and 
then multiply by 5/9. In the other direction, to convert a temperature 
in degrees Celsius to the Fahrenheit scale, we must first multiply by 
9/5 and then add 32 degrees. The Celsius scale is now used everywhere 
outside the United States, so only Americans need to remember these 
formulas.

- Doctor Sarah, The Math Forum
  http://mathforum.org/dr.math/