Origins of the Fahrenheit Scale

Date: 11/20/98 at 15:51:13
From: Stephen Valentine
Subject: Fahrenheit temperature scale

Why did Fahrenheit choose 32 as the freezing point of water?

Date: 11/20/98 at 17:05:46
From: Doctor Rick
Subject: Re: Fahrenheit temperature scale

Hi, Stephen.

Historical "why" questions are notoriously difficult to answer. I have 
seen several accounts of Fahrenheit's work that differ in some details. 
Britannica (1970 edition) in the article "thermometry."

Daniel Fahrenheit did not choose 32 degrees as the freezing point of 
water, nor did he choose 212 degrees as the boiling point of water. 
Both these numbers are just the values he measured after he had based 
his scale on two other "fixed points."

The lower fixed point was the lowest temperature he could reach by 
mixing water with common salt (the way old-fashioned ice cream freezers 
work). This temperature he called 0 degrees.

The upper fixed point that Fahrenheit chose was the temperature of the
healthy human body. This he set at 96 degrees. Why 96? you ask. Why 
would he come so close to 100 and not decide to go decimal? The 
encyclopedia says that he originally divided the interval into 12 
degrees and later divided each degree into 8 parts. 

To us, decimal seems the only way to go. I can only speculate that, in 
the days before calculators, people found fractions more convenient 
than decimals. Numbers like 12 and 16, and 12 * 8, are more useful than 
10 in the world of fractions because they have more divisors. For 
instance, 100 is divisible only by 2, 4, 5, 10, 20, 25, and 50, while 96 
is divisible by 2, 3, 4, 6, 8, 12, 16, 24, 32, and 48.

Fahrenheit measured the boiling point of water with his new mercury
thermometer to see if it was stable. (Previous thermometers using 
alcohol couldn't measure that high.) When it was found that this 
temperature was a stable 212 degrees on Fahrenheit's thermometer, this 
was made the upper fixed point of the Fahrenheit scale. Likewise, the 
lower fixed point was moved to the freezing point of pure water, which 
was 32 degrees on Fahrenheit's thermometer.

Maybe you are wondering, isn't body temperature 98.6, not 96 degrees? 
Well, body temperature isn't quite that precise, but yes, Fahrenheit's 
measurements weren't quite linear. After the scale was fixed at 32 and 
212 degrees rather than 0 and 96 degrees, more accurate, linear 
thermometers were developed and it turned out that normal body 
temperature is a few degrees higher than Fahrenheit had measured.

Take a look at this article from the Alaska Science Forum: "Daniel 
Fahrenheit, Anders Celsius Left Their Marks," by Ned Rozell:

   http://www.gi.alaska.edu/ScienceForum/ASF13/1317.html   

It's interesting but it differs in some details from the Encyclopedia 
Britannica's account. This just tells us that history is more 
complicated than we (especially mathematicians?) would like it to be. 
It's still fascinating to try to put ourselves in the mindset of 
another century and understand the choices they made.

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