# TemperatureMeasurements

I'm sure you know the difference between hot and cold water. If not, stir your iced drink and your boiling soup a while with your finger and you shall know. You probably also know the difference between a cold piece of iron an a hot one. If not, just touch the exhaust pipe of your car after you went for a long ride.
It's easy to know if an inanimate object is hot or cold. You find out by touching it. With animated objects it might be more difficult. How hot is that lady over there? Finding out by touching might not be such a good idea.
How about measuring "hotness" or "coldness"? That means to put a number on coldness and hotness and call it temperature.
Typically you must now touch the object with some device that gives you a response on some scale.
You now have two problems:
1. What kind of device could do that?
2. What kind of scale should I choose?
Despite appearances, the two problems are completely unconnected. That becomes clear immediately if you look at the simplest device for measuring temperature: the good old mercury (Hg) thermometer. Mercury (or other liquids like alcohol) expand when becoming hot, and the top of a column of the stuff contained in a thin glass capillary will climb up if the temperature goes up.
OK, we solved the first problem. We now have some device that responds to temperature by moving a "pointer" (the top of the column) up and down a scale.
But what kind of number do I put on the scale next to the capillary? Is there some universal principle that I can use for guidance?
Yes there is. For questions like this, the universal principle is:

In this case she will weasel a bit. She might mumble that the best way would be to express temperature in terms of energy, but that you won't get this, and that the next best thing would be to use the so-called absolute temperature scale, which gives temperatures in "Kelvin (K)". It's just Kelvin, not "degree Kelvin"; no oK.
However, if you press her a bit, she will admit that in everyday life she actually uses centigrades (oC), degree Fahrenheits (oF) or whatever else she grew up with (if she is of French origin it might be Réaumur (oR) but we won't hold that against her).

 Temperature measuring devices called thermometers

Both thermometers measure the same thing ("temperature") but with different scales: degree Celsius or degree Fahrenheit.
The guys who invented the first thermometers (there were only guys, female scientists hadn't been invented or discovered yet) had different ideas about how to fix a scale.
The Swedish astronomer Anders Celsius in 1742 assigned 0 °C to the freezing point of water and 100 °C to the boiling point. In between the scale was subdivided in 100 parts (defining a temperature difference of 1 °C), which allows to extrapolate to temperatures above 100 °C and below 0 °C. 1)
The Celsius scale is the scale almost everybody uses today (even the French).
Daniel Gabriel Fahrenheit, a German from Danzig, begged to differ in 1714. He picked as the zero point (0 °F) of his scale the low temperature of the stiff winter 1708/1709 in Danzig (-17,8 °C on the Celsius scale), because he wanted to avoid negative numbers. For rather obscure reasons 2) he took 32 °F for the freezing point of water, which automatically leads to 96 °F as another fixed point for the body temperature of humans.
From a scientific point of view the Fahrenheit scale is problematic because its fixed points are not well defined. From a practical point of view it is a great scale because 0 °F - 100 °F defines about the extremes of temperature humans may experience. You know that 0 °F is lousily cold and 100 °F is scaldingly hot.
Fahrenheit is still the official scale of the United States, Thailand and Belize; in Canada it is retained as a secondary scale. The rest of humankind uses °C or "centigrades".
René Antoine Ferchault de Réaumur proposed a scale with 0 °R as the freezing point of water and 80 °R for the boiling point. Why 80 and not 100 or 72,6 or whatever was a kind of mystery until I found the explanation. 3)
My own explanation was that the French have trouble counting beyond 20, and going up to 100 was just too challenging. 80, in French is "quatre vingt", i.e. 4 times 20; 90 is "quatre-vingt-dix", i.e. 4 times 20 and 10. Counting just gets too complicated above eighty.
There are several other scales, all but forgotten, and I won't go into this anymore.
Instead we give a look at the scientifically important Kelvin scale or absolute temperature scale. It rests on the tremendous insight that there is a lowest temperature, a coldness that cannot be surpassed, a natural absolute zero point of temperature. You can't do better than that to define 0 K or zero Kelvin.
Going up is done with intervals borrowed from the Celsius scale. A difference of 10 K is the same difference between some two temperatures as 10 oC difference.
There is no particular reason for using Celsius, except that it was convenient for the majority of scientists who were used to the Celsius scale by their upbringing.
The Kelvin scale is so important in science because absolute temperature measured in Kelvin (and then always abbreviated T) can go right into equations and formulae.
Here is a conversion diagram for the four scales discussed:

 The following footnotes come all from this article: W. Dreyer, W.H. Müller and W. Weiss: "Tales of Thermodynamics and Obscure Applications of the Second Law", Continuum Mechanics, Thermodyn. 12 (2000) pp 151 - 184 1) 2) 3) .

Glossary

Invention or Discovery?

History of Carbon

4.3.1 Nirvana for Crystals

2.1.1 Bang it!

Units of Length, Area, and Volume

4.4.1 Perfect Crystals and the Second Law

6.1.1 It Takes Two to Tango

4. Mercury

The Second Law

8.3.3 Bang it!

© H. Föll (Iron, Steel and Swords script)