What is Zero Fahrenheit, Anyway? Is it really the freezing point of sea water?

Slug Signorino

I came across your column on what zero means on the Fahrenheit scale. You blew it. You said that, unlike 32 or 212 degrees, zero degrees corresponded to nothing in nature—it was merely an arbitrarily assigned number. It isn’t. It’s the temperature at which seawater will freeze. Of course it’s an approximation, because the freezing point of saltwater varies based on salinity, but zero degrees is a rule of thumb. I’m not playing gotcha here—just battling misinformation wherever it rears its ugly head. —Richard Forte

Then let me assist you in your battle, Richard: You’re wrong.

I admit you’ve got a lot of company. Wikipedia takes your side, as does at least one college physics textbook. But close examination makes it reasonably clear the seawater explanation derives from a misreading of the evidence.

In my 1989 column I said that Daniel Gabriel Fahrenheit based his system of temperature measurement on an earlier scale devised by Danish astronomer Ole Roemer. Roemer, I said, had set zero arbitrarily—his main consideration was that it was colder than the temperature ever got in Denmark, because he didn’t like using negative numbers in his weather logbook.

Roemer’s scale had 7.5 as the freezing point of water and 22.5 as body temperature, in those days called “blood heat.” To get rid of the awkward fractions, Fahrenheit did some multiplication, eventually winding up with 32 as the freezing point and 96 as body temperature. (Boiling point initially didn’t figure in his scheme.)

I said that when Fahrenheit was set to demonstrate his system to London’s Royal Society in 1724, he worried it would look odd if zero on his scale was untethered to reality, and thus had to concoct a rationale. Here’s what he wrote in the paper he presented:

“The division of the scale depends on three fixed points, which can be determined in the following manner. The first is found in the uncalibrated part or the beginning of the scale, and is determined by a mixture of ice, water and sal ammoniac [ammonium chloride], or even sea salt.” The “or even” part (the Latin phrase is vel etiam [salis] maritimi) is a giveaway—the freezing point of seawater was an afterthought. Fahrenheit underscores this as he continues: “If the thermometer is placed in [the water-ice-ammonium chloride] mixture, its liquid descends as far as the degree that is marked with a zero. This experiment succeeds better in winter than in summer.”

Think what this means: The method supposedly used to determine zero on Fahrenheit’s scale doesn’t always work. Who would be foolish enough to invent a temperature scale that wouldn’t permit thermometers to be reliably calibrated? In contrast, the freezing point of fresh water, as manifested in an ice/water mixture, is constant for practical purposes, making it a dependable benchmark. It seems obvious the ammonium chloride/seawater procedure had been invented after the fact to provide a physical correlative for a point originally chosen for other reasons.

But don’t take my word for it. In a letter Fahrenheit wrote on April 17, 1729, he says that when he visited Roemer in 1708, he found several thermometers being calibrated by standing in water and ice. These thermometers were then heated to body heat, and “after [Roemer] had marked these two points on them all, half the distance found between them was added below the point of water and ice, and this whole distance was divided into 22.5 parts, beginning at the bottom with 0, arriving thus at 7.5 for the point of water mixed with ice, and 22.5 for the point of blood heat.”

There you have it. Fahrenheit, following Roemer, simply determined the distance between the marks for the freezing point of water and body heat on his glass thermometers (64 degrees, in the scale he would ultimately develop), measured off half this distance (32 degrees) below the freezing point, and called that zero.

Recounting this story in a 1991 article, R.J. Soulen of the U.S. Naval Research Laboratory writes: “The zero on this scale had no fundamental meaning, following the tradition of others. Fahrenheit chose to define a zero below the coldest temperature likely to be encountered by everyday use of his thermometers.”

As I said. To be fair, Fahrenheit wasn’t the only early scientist to come up with quirky calibration procedures: Robert Boyle proposed that thermometers be calibrated to the temperature of congealing aniseed oil; Joachim Dalencé suggested pegging them to the freezing point of water and the melting point of butter; and the Encyclopaedia Britannica thought a useful temperature reference point was “water just hot enough to let wax, that swims upon it, begin to coagulate.”

At least these benchmarks were practical. Try calibrating your thermometer using the standard proposed by 19th-century astronomer Charles Piazzi Smith, who nominated a scale set to “the mean temperature of the King’s Chamber at the center of the Great Pyramid of Giza.” Great idea, Chuck. —Cecil Adams

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