What I Learned Today

If you had to pick reference points to define a new temperature scale, what would you use? If you were Daniel Gabriel Fahrenheit, a glassblower and chemist, you picked: the equilibrium temperature of a specific brine, the temperature at which water freezes, and the temperature of the human armpit. Apparently, he used his wife as his calibration reference. The brine is interesting: a frigorific mixture of ice, water, and ammonium chloride. Frigorific indicates that the mixture goes to an equilibrium temperature no matter what the initial temperatures of the components were. Water + ice is a frigorific mixture that goes to 32 F (or 0 C).

Next, what numbers do you assign to these reference points? Fahrenheit assigned 0 F to the brine, 32 F to the ice, and 96 F to human body temperature. The seemingly arbitrary numbers arose because Fahrenheit modeled his system after the Romer scale, which assigned 0 degrees to brine and 60 degrees to boiling water, which put the freezing point of water at an awkward 7.5 degrees. Fahrenheit multiplied everything by 4, got 96 degrees for the human body, and then fudged his definition of a degree (which just means a “step”) a bit so that the freezing point of water would be exactly 64 degrees less than body temperature (so, 32 F). Why? So that his thermometers could be easily marked between those two points — 64 is a power of 2 and that means it’s easy to get equal divisions by repeatedly marking the midpoint between existing marks.

Fahrenheit published his scale in 1724. Later, the scale was revised so that there would be exactly 180 degrees between the freezing and boiling points of water (180 being a number that is divisible by a conveniently large number of numbers). This pushed the human body temperature up, on the Fahrenheit scale, to 98.6 F.

That’s all very interesting, but you have to wonder: once you’ve invented a new scale, how do you persuade others to use it? I’m wondering what kind of marketing Fahrenheit employed, other than his presentation at the Royal Society of London. Clearly, it was quite effective… until Anders Celsius came along 20 years later.