Fahrenheit and Celsius – Trapped in a 10-sided box

The Fahrenheit temperature scale is a wonderful history lesson and a beautiful triumph of design. It’s just lost to our modern sensibilities.

This post is about Fahrenheit, the temperature scale that no one uses, except notably the United States. Yes, I’m an American, and I want to say some nice things about Fahrenheit. Wait wait! Don’t go! It’s not what you think! I’m very much not saying Fahrenheit is better than Celsius, it’s not. Indeed, while we’ve got bigger fish to fry right now, standards are important and the US should use SI ¹

I love talking about Fahrenheit. It offers an excellent distillation of history, the nature of scientific measurement, and even a lovely illustration of creative, outside-the-box-thinking.

Creativity and temperature? What gives?

When someone first encounters Fahrenheit, they generally recoil at the fact that the boiling point of water is 212 degrees. “212 degrees? Why such a weird number? Why not a nice number, like 100 degrees?”

That’s a fair question but it’s also a little presumptive. “Why not my way?” isn’t really giving this new entity the benefit of the doubt. Maybe there’s a reason behind the choice of number?

So, here’s an alternative question that almost no one would ask when encountering Fahrenheit, but perhaps they should:

What has a temperature of 100 degrees Fahrenheit ?

Did you ever think to ask that? Do you know the answer? You. You have a temperature of 100F.

• Now, hold up. It’s more complicated than that: For one, Fahrenheit didn’t pick 100F exactly ². The point is he chose human body temperature for a reference. Is that a better choice than the boiling point of water? No, it lacks precision. But he had some excellent reasons in his era for it. And nowadays we never consider the possibility of a reference point except boiling or freezing water.

This brings up my second point on creative thinking. Sometimes it’s worth asking a painfully obvious question because you can learn a lot if there’s a very not-obvious answer.

What is the purpose of a temperature scale?

A temperature scale is used to record and communicate temperatures, right? You need a common language, a standard scale so everyone can reckon temperature. Right? This is what Celsius was designed for (and the metric system more generally).

This is not what Fahrenheit was designed for.

Fahrenheit was designed for making accurate thermometers when you lack the tools for making accurate measurements.

Think of the historical context. It’s 1724. Precise rulers are rare. Ice may be hard to come by and it takes a lot of firewood to boil water. There probably won’t be any distilled water. What can you do?

Fahrenheit’s choices when developing his scale were shaped by concerns that are entirely irrelevant today but were critical in his time. The Fahrenheit scale is not bad or weird, it’s obsolete.

Consider placing markings on the glass of the mercury-filled thermometer (which you just invented) after you’ve marked the reference points. You want these to be evenly spaced so numbers read from that scale are accurate. Maybe you have a straight edge, a compass, and a piece of string, but no accurate ruler. How can you accurately divide a line segment into 10 or 100 equal lengths?

Under these circumstances, would you rather divide that line segment into 8 lengths instead of 10?

What difference does 10 vs. 8 make? A lot, because $8 = 2^3$. All you have to do is learn to find the midpoint accurately ³, and then do this repeatedly.

(We take base-10 in SI (metric) for granted now, but it was extremely radical and ahead of its time. It was too hard to switch from base-2 to base-10 until accurate tools could be manufactured at scale. Two hundred years ago, a base-10 system, which is so natural to us now, was actually an argument against SI. Now, of course, the US persists with “standard” out of just plain stubbornness.)

Lastly, Fahrenheit’s other choices also reflect concerns over accuracy ⁴. For one, he decided not to use the freezing point of water as the zero reference. Instead, he chose 0F as the temperature of a brine—a mix of water, ice and ammonium chloride—because it is extremely stable thermally, more so than just water and ice. He also picked 32F for the freezing point of water because again dividing into a power-of-two number of line segments is easy to do accurately ⁵. And the fact that Fahrenheit is convenient for weather is no coincidence—Fahrenheit rejected using the boiling point as a reference because it was unsuitable for many meteorological applications.

Takeaway

So what’s the lesson here? Really, it’s Chesterton’s Fence: when you encounter an idea that doesn’t comport with your view (212 degrees? Why not 100?), try to think outside your box (If 212 is boiling, what is 100?) and see if there’s a reason behind it. Take a pinch of history and humility—you might learn a lot.