The Origins of Fahrenheit

Date: 09/16/98 at 17:24:46
From: barbra
Subject: Fahrenheit

Why did Fahrenheit decide to make 32 degrees his freezing point and 
212 degrees his boiling point?

Date: 09/17/98 at 12:08:04
From: Doctor Peterson
Subject: Re: Fahrenheit

Hi, Barbra. It is sort of puzzling, isn't it? But what really happened 
is that he used a different pair of temperatures to define his scale, 
and when he measured the freezing and boiling points of water on his 
scale, they turned out to be 32 and 212. I did a little research and 
discovered that the explanation isn't quite as simple as I had thought.

His choice for 0 was the coldest temperature he could attain in his lab 
(since negative temperatures are inconvenient, and he didn't know that 
there is an absolute zero which he could have used - one we now use 
for the Kelvin scale). He used the temperature of a certain ice and 
salt mixture, like what we sometimes use to freeze ice cream at home.

His other reference point was "normal" body temperature. Often people 
simplify the story and say he set this to 100. Well, not quite: he 
actually used 96 for body temperature, either because it is 8*12, or 
because 96 - 32 = 64. Either way, it made the scale easy to divide. 
We call body temperature 98.6, but in fact body temperatures vary a 
lot, and whoever he measured to get his 96 may have had a low normal 
temperature, or it may simply have been before his scale was adjusted 
later. Actually, 98.6 degrees Fahrenheit is exactly 37 degrees Celsius, 
so I suspect it is really just an approximation in Celsius, which when 
translated to Fahrenheit sounds much more precise than it really is! 
(The range of 36.5 to 37.5 translates to 97.7 to 99.5.)

It appears that the scale was refined over a period of time, trying out 
different measurements as the real standards, including the freezing 
and melting points of water. By the time it was standardized, body 
temperature was not part of its definition. Instead, the value used for 
the boiling point of water was set at 212, so that the difference 
between 32 and 212 was a neat 180. (This makes the conversion to 
Celsius much easier than it could have been.)

So maybe the best simple answer is that 32 and 212 were chosen so that 
salt and ice would be about 0 and body temperature would be about 100.

The messiness of this story is a good illustration of the fact that 
science doesn't pop out of scientists' heads full-grown, but involves 
lots of wrong turns and bad ideas before an orderly scheme such as the 
Celsius scale is born. (And even Celsius has undergone some changes, 
but I won't get into that.)

Here's another discussion of this question in our Dr. Math archives:

   http://mathforum.org/dr.math/problems/hokanson7.26.97.html   

and some interesting (and contradictory) explanations elsewhere:

  From the Alaska Science Forum, by Ned Rozell:
   http://www.gi.alaska.edu/ScienceForum/ASF13/1317.html   

  Fahrenheit's Thermometer, by J. B. Gough
   http://www.rci.rutgers.edu/~jup/metric/fahrenheit.html   

  What Marilyn omitted about the Fahrenheit Thermometer by Herb Weiner
   http://www.wiskit.com/marilyn/fahrenheit.html   

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