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The Fahrenheit scale is defined by fixed points on the scale. What interests me is the apparent arbitrary chosen numbers in these fix-points.

First wikipedia wites from 32 to 212. -and later in the History part we have from 0 to 32 to 96(based on a quadrupling of 7.5 to 22.5 to 60). I am stille confused.

Does any of the numbers make sense in an other numeral system? Did Rømer or Fahrenheit use their lucky numbers or what is going on?

Edit: I am not interested in what solutions or objects, that is used for the fix-points. I am asking what the logic is behind the chosen numbers?

hpekristiansen
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3 Answers3

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According to the same Wikipedia article you cite,

...the zero point is determined by placing the thermometer in brine: he used a mixture of ice, water, and ammonium chloride, a salt, at a 1:1:1 ratio. This is a frigorific mixture which stabilizes its temperature automatically: that stable temperature was defined as 0 °F (−17.78 °C). The second point, at 32 degrees, was a mixture of ice and water without the ammonium chloride at a 1:1 ratio. The third point, 96 degrees, was approximately the human body temperature, then called "blood-heat"

According to a letter Fahrenheit wrote to his friend Herman Boerhaave, his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer's scale, brine freezes at zero, water freezes and melts at 7.5 degrees, body temperature is 22.5, and water boils at 60 degrees. Fahrenheit multiplied each value by four in order to eliminate fractions and increase the granularity of the scale.

Rømer's choice of 60$^\circ$ as the boiling point of water makes sense if you consider the fact that Rømer was an astronomer, so 60 has special significance. So really it was Rømer who pioneered non-decimal-based temperature scales, Fahrenheit was just following suit.

Community
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Kyle Kanos
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The story is this, as much as I remember. Fahrenheit chose the zero point on his scale as the temperature of a bath of ice melting in a solution of common table salt (a routine 18th century way of getting a low temperature). He set $32^{\circ}$ as the temperature of ice melting in water. For a reproducible high point on the scale he chose the temperature of the blood of a healthy person (fun fact: in this case the healthy person was his wife) which he measured in the armpit and fixed it at $96^{\circ}$. After Fahrenheit died, his successors used the boiling point of water to calibrate the thermometers. And they set it at $212^{\circ}$ such that it retains the size of Fahrenheit's degree.

vnb
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This answer has nothing to do with physics, but I was taught in grade school - and, yes, this was in an actual elementary school text - that the Fahrenheit thermometer was based on the coldest and hottest days in 1714 in Holland where he lived. I have never been able to verify that, so I assume it was false, but the temperatures of zero and 100 do represent about the extremes that one can expect to experience in Western Europe where I have lived for several years now.

David
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