|MadSci Network: Physics|
Ulrich this is an interesting question.
The Celsius scale uses the freezing and boiling points of water as its fixed points. In the 17th century they used the lowest attainable temperature (achieved by mixing salt with water) as their Zero degree and the temperature of the body as the other fixed point.
At this time a man called Roemer had invented a scale which Daniel Farenheit used as the basis for his own scale. Farenheit did not understand Roemer's scale properly, but he took it to mean that the freezing point of water was at 7 1/2 degrees and that the body temperature was 22 1/2 degrees.
He put Roemer's scale on one of his thermometers and then sub-divided each degree into four parts to make it more accurate. This now gave him a scale where the freezing of water was on the 30th sub-division and the body temperature was on the 90th sub-division.
Farenheit did not like the fact that the fixed points were not exactly on a major division on Roemer's scale so he altered it. He set the freezing point at 8 degrees and the body temperature at 24 degrees. When he sub-divided each degree the freezing point was at 32nd sub-division and the body temperature was at 98th sub-division. This is very close to the modern Farenheit scale.
At some stage the idea of using the boiling point as a fixed point gained more acceptance. Now Farenheit reported in an article that on his scale the boiling point of water was actually 212 degrees. This was not accurate but that did not matter. Everybody who used the Farenheit scale set the fixed points at 32 and 212 degrees. When body temperature was measured using this final scale it was found to be 98.6 degrees Farenheit.
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