This is the Season to Care about Temperature
by Thomas Oestereich

In winter, we need heating, in summer, we use our air conditioners to cool down. Seems we are never content with the temperature nature provides. What temperature is it there where you are quite now? I mean - you are probably reading this indoors at your computer, and the temperature could be something like 68°F or 20°C or 16°R or ... what? Have never seen °R? Stands either for "degree Reaumur" or for "degree Rankine". All these units are named after scientists, and René Antoine Ferchault de Réaumur, obviously a Frenchman, introduced his scale in 1731. It's practically not in use any more. I have heard that some US engineering bureaus do use the Rankine temperature scale.

What do these scales have in common? First and most importantly: They measure temperature, whatever this may be. Second: They are "linear measuring scales" each defined in a way so that measurements can be reproduced everywhere and yield the same results under same conditions. To achieve that, either a point and a slope, or two points have to be defined, as it is the case with any straight line (see your maths lessons).

First to the case of good old Daniel Gabriel Fahrenheit. In a cold winter 1708/1709 in his home town Danzig, part of Germany at the time (today Poland), he used the (outdoors!!) temperature of those days as one of two defining points. He was convinced that it could never become any colder than that, so he assigned the value ZERO to it, 0 F. To be exact, he chose a temperature a little below the outdoors temperature, but one which he could reproduce independently of the weather: A cooling mixture of ice, water, and ammonium chloride has always the same temperature and can always be reproduced.

Well, it became colder, not only in Danzig, so negative values have nevertheless to be used with that scale. But now to the other defining point, and what is in between? As the second point Fahrenheit chose his wife's body temperature in her arm pit and set it to 96° (after a later recalibration of the Fahrenheit temperature scale, the normal body temperature today is 98.6°F, corresponding to 37°C). And then with these thermometer definitions, he found the freezing point of water to be 32° and the boiling point 212°.

And between the two points? - A small glass bulb connected to a glas tube was filled with mercury or alcohol. By thermal expansion, the liquid enters the tube and fills it to some point, depending on the temperature. The distance between the two defining points was subdivided into 96 parts or rather degrees. Quicksilver or alcohol? All the same? - Not really. But for the precision which was required back then, the small differences in linearity of expansion between the two liquids did not matter. You see, at the time, making a thermometer was an art. You could not just go to Walmart and buy one. There was no Walmart around. But then, even today for a household thermometer, if it is still based on a glass tube with liquid instead of an electronic device, it does not really matter whether red or blue colored alcohol or quicksilver is the liquid. For precision measurements, quicksilver is the better choice.

Fahrenheit worked on thermometers for many years, and 1724 he finally publicly proposed the scale which now carries his name in an article published in the "Philosophical transactions, London" of 1724. But there he described his higher defining temperature point as... "A third point, designated as 96, is obtained if the thermometer is placed in the mouth so as to acquire the heat of a healthy man." His thermometers were crafted with a main scale of twelve divisions, each parted in half (24) and again in half (48) and again in half (96°). The "second point" was the temperature of freezing water, the "first point" the temperature of the cooling mixture. As only two points are really needed, it is suspected that he had dropped the "first point". Possibly he traded his the "third point" for the something else and rather calibrated his thermometers with the freezing and boiling temperatures of water, 32°F and 212°F.

The two other temperature scales I have mentioned above use the freezing and boiling points of water. While the Swede Anders Celsius divided this difference into 100 steps or rather degrees (published 1742), Reaumur divided it into 80° only.

But I do like the idea of Fahrenheit: Use a zero point low enough so you don't need negative values for the temperature. Although he failed.

But HOW low do I have to set my zero point so negative temperature values won't occur at all?

What do you think? Is there a limit below which temperature cannot fall, so when we take that as zero, we are safe from negatives? Just compare with other instances: Negative money, debt. Is there any limit? Is there a point below which nobody can fall? Practically: Yes. One day, nobody will lend you anything any more, and then you cannot go into any deeper debt. But this limit is very personal. We have all heard stories about people who managed to get millions from the bank with very shaky securities. Their debt limit obviously was much, much further into the negatives than ours who do not get a check cashed without them taking a portrait photo.

So: No, in most cases where negative values play a role, you will not find a lowest value. There's no natural zero point of debts, and none for practically anything else negative.

Now the big surprise: Temperature is different. You didn't expect this, did you? There IS a natural zero at -273.15 C or -459.67 F. This has something to do with the very nature of temperature; it is the density of heat energy a substance contains. When this energy is removed completely, then the temperature cannot drop any further. Said easily, but done ... well, the absolute zero of temperature cannot be reached, but it can be approximated. Every ten years one order of magnitude, roughly speaking. Now we are somewhere in the billionth degrees. So perhaps we should rather use the logarithm of the values our current concept of temperature delivers. This would be as good a measure, but one whose values go to negative infinity when our actual concept of temperature goes towards absolute zero.

By the way, there has been a definition of yet another temperature scale based on this absolute zero. It is called the Kelvin scale after a physicist William Thomson who died as Lord Kelvin, raised to peerage by the British queen Victoria for his scientific achievements. With that scale which he proposed 1848, the size of the steps or rather degrees is defined to be equal to that of the Celsius scale and zero is the absolute zero of temperature. So the freezing point of water is 273.15 K, the boiling point 373.15 K. Note: No degree sign ° with Kelvin. Because it starts at "absolute zero", it's called an absolute temperature scale. Incidently, the Kelvin scale has been accepted 1960 as the "SI" temperature scale, meaning the one to be used in science.

Just to make things more complicated, a "Rankine" scale has additionally been defined with the degree size equal to that of the Fahrenheit scale and zero at the absolute zero. So 459.67°Ra is equal to 0°F. Rankine was a Scottish engineer and physicist and proposed his scale in 1859. As it is another absolute temperature scale, some people don't put a degree sign ° here either. Others use °R and risk that degrees Rankine could easily be mistaken for degrees Reaumur °R. I don't know anybody who uses any of the two, but that may be due to the fact that the number of my acquaintances is limited.

For your normal life, the conversion between Fahrenheit and Celsius temperatures may be the one which is most important. You can use the following formulas, written in a mathematically incorrect way (but easy to remember):

°C = (°F - 32) / 1.8 °F = (°C x 1.8) + 32

or you simply use the conversion resource here: http://physics.global-momentum.net/resources.php

All the best

Thomas Oestereich<

After 20 years of being a physicist, Dr. Thomas Oestereich turned from scientific research to a life as an author and editor of his home school curriculum. He now seeks to help his readers gain access to the insights of physics. Bringing the achievements of science to a larger public, he hopes to finally contribute to a better knowledge of the options and constraints of decision making in our democratic society. http://www.physics.global-momentum.net Contact the author, Thomas Oestereich , at speed-up@gmx.net
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