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/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994-2013 The Math Forum