# What is the logic behind the Fahrenheit scale?

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?

• Zero was the coldest temperature achievable with salt + ice and 100 was the human body. – Brandon Enright Nov 30 '13 at 3:19
• @Brandon Enright: So 0 to 100 - that makes sense. Do you have a reference for this? It is not what is written on wikipedia. – hpekristiansen Nov 30 '13 at 3:40
• There is no logic behind those numbers (at least as we see it now). People in that time chose "events" that were at hand and reproducible. Take the case of Celsius. When he first devised his scale, he set the freezing temperature of water as $100^{\circ}$ and $0^{\circ}$ as the boiling temperature. Where's the logic in that? – vnb Nov 30 '13 at 3:46
• @vnb: We use the decimal system both now and then. 100=10^2 – hpekristiansen Nov 30 '13 at 4:00
• That I know. But from what books I've read, I can't tell you why Fahrenheit or the others used to divide the temperature scale in that way. My intuition tells me that it might have something to do with the degrees of the circle. The difference between 212 and 32 is 180. Like setting the temperatures at opposite sides of the diameter. I don't know, I'm shotting in the dark right now. – vnb Nov 30 '13 at 4:08

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.

• Thank you - now I see it. Rømer liked to divide the scale into 1/60 - like (arc-)minutes and seconds. – hpekristiansen Nov 30 '13 at 10:02

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.

• This also meant water boiling and freezing were 180deg apart which seemed sensible. Remember before calculators - numbers easily divisible (like 60 and 12) were more useful than powers of 10 – Martin Beckett Nov 30 '13 at 3:33
• This is what is written on Wikipedia. It is not what I am asking. I will make an edit to make my question more clear. – hpekristiansen Nov 30 '13 at 3:36
• Please do. I thought you asked what is the significance of these numbers and how people got to them. – vnb Nov 30 '13 at 3:38
• What we were taught when I was in school was that Fahrenheit initially calibrated his scale such that zero was the coldest temperature he could achieve with a (liquid) salt water solution (at the time) and 100 was human body temperature. By that scale, other reproducible temperatures such as the freezing point of pure water occured at inconvenient values, so the scale was adjusted, producing the scale we have today. Although it is an account of the origin, it doesn't explain the evolution to the scale as we know it today. – Anthony X Nov 30 '13 at 4:28

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.