Fahrenheit Scale - what's the basis?

Marshall Street Starbucks - Syracuse, NY

Curiosity made me look up the logic recently. It was surprising!

What I mean by the title is that I know what the Celsius scale is based upon. Water freezes at 0C, boils at 100C, and using these two round values as base points, the scale is calibrated. Sounds reasonable.

Now, the Kelvin scale. The scale is the same as Celsius, but with an offset. That is, each difference of 1K is equal to 1C, but the base is not the freezing point of water, but absolute zero. In other words, any temperature in this universe, however cold, would be non-negative on this scale. Zero Kelvin is when all motion ceases (not yet attained experimentally but about 1E-9K has been reached). Very clear reasoning.

There's other scales but they do not appear to be very frequently used, so I won't comment on them for now.

This brings us to Fahrenheit. Water freezes at 32F. Water boils at 212F. Normal body temperature is 98.6F. Absolute zero is -459.67F. So what really is the basis for this scale? There should be some "round" number that means something. Why choose seemingly arbitrary numbers? We know what zero is in Celsius and Kelvin. What really is "Zero degrees F", in  other words, what is the basis for this scale?

The only thing I could think of was that perhaps 100F is a likely threshold for a (serious) fever. 98.6-100F may not be a cause for much concern, likely a simple cold/flu. Although I must admit, this was not strong enough a reason, and I couldn't think of any other round number, in order to calibrate.

It so happens that this scale is still popular partly because of the following "intuitive" sense of these numbers, that were easy to remember. 50F is a pleasant day. 100F is on the threshold of being too hot. 0F is borderline very cold. And so on.

But none of all this explained the logic behind creating the scale. I mean 99F is very hot as well. 1F is chilly too. 55F is even more pleasant. 100.4F is also a reasonable fever.

As it turns out, the scale just happens to produce values this way. The reason behind creating the scale as it is was very different. Fahrenheit, in 1724, determined the zero as the freezing point of a mixture of ice, water and ammonium chloride (in equal rations), essentially a kind of brine. At the time, this temperature was considered widely as the coldest possible.

32F was the freezing point of ice and water (also mixed in equal ratios). The number 32 came about as Romer's scale (proposed in 1701) was modified by Fahrenheit to avoid fractions (he multiplied all values from Romer's scale by 4). This new scale underwent further minor refinements by scientists which led to the scale being the way it is today.

OK, now the next (obvious) questions - 1. Why ammonium chloride and not other salts? 2. Why was Romer's scale set to 7.5 for water/ice freezing mixture?

1. Ammonium chloride is a commonly available salt used in forming Frigorific mixtures, i.e. a mixture where the final equilibrium temperature reached by the mixture is fixed, no matter what the temperature of the constituents were, initially. I assume this particular salt was used (and not some other salts which also produced such mixtures) since it was commonly available, but I'm unsure.

2. Romer set 0 in the same way as above, the freezing point of brine. The boiling point was set at 60 degrees. Why 60? I tried searching on the net, but found no convincing answer. Well, it's a round number. And the freezing point of water on this scale was 1/8 of the way, at 7.5 degrees.

So there's my understanding of the scale. The availability of ammonium chloride abundantly, its contribution towards forming frigorific mixtures, a misunderstanding of what the coldest possible temperature was, and an (arbitrarily?) chosen boiling point of 60 degrees by Romer led to the scale that is still so popular today!

In my opinion, metric systems are the way to go!