Why water freezes at 32F

Why water freezes at 32F

Modern folks often consider Daniel Fahrenheit's thermometer scale less "logical" than Anders Celsius' scale in which the difference in temperature between freezing and boiling water is set to 100 C. Fahrenheit, in his older scale, originally set the difference between freezing water and human body temperature equal to 64 F.

Why 64? Imagine making a thermometer: you fill the tube with your working liquid (mercury, or in Fahrenheit's day alcohol). You put it one of your references (say freezing water) and mark the level. Then you put it in another (in your mouth, for Fahrenheit's scale) and mark the level again. Now you need to divide the space between into equal divisions. If you have to make 100 divisions, you need a precision steel ruler. But if you have to make 64 divisions, you just need a compass and straightedge, because you just need to divide the distance exactly in half 5 times (2^6 = 64).

Furthermore, Fahrenheit took advantage of an interesting coincidence to set the freezing point of water at 32 F, and assign a third scale point, the temperature of a stable mixture of ice, salt and water, to 0 F. Since 2^5 = 32, this distance, too, can be marked off very easily with compass and straightedge.

Hence Fahrenheit's scale lent itself far easier to the manufacture of accurate thermometers at a time when precision instruments were very expensive.

Daniel Stern