|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:
|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
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).
|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
The Celsius scale is the scale almost everybody uses today (even the French).
Gabriel Fahrenheit, a German from Danzig,
begged to differ in 1714. He picked as the zero point
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
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:|
|I must now discuss all those
questions that accumulated in the back of your mind. Let's bring them out:
|1. How do we measure (extreme) temperatures? There are many ways; we will only give some of them a quick glance.|
|Most simple and wide spread is the
use of a resistor that changes its
electrical resistivity in a well defined way with temperature. Platinum (in a
thin layer, so you only need tiny amounts) does just that. And no, I won't go
into why Pt (and just about everything else) changes its resistivity with
That's great because this allows you to generate an electrical signal, that can be processed directly by the "electronics inside" that run about everything today. Platinum is also a great material for this because it will not corrode and can take high temperatures because its melting point is 1.772 oC (3.222 oF)
|Quite simple and widespread are also
"thermocouples", a junction
between two different materials. If that junction is at a temperature different
from that of the rest of the material, a voltage develops that is indicative of
the temperature (difference) and that you can easily measure with a voltmeter.
An no, once more, I will not even try to attempt to explain why that happens.
Thermocouples are widely used at high temperatures and when precision counts.
|What are we going to do at really high temperatures, when pretty much
everything would be molten? Easy. We look at the light emanating from all hot bodies.
What you and just about everybody else knows is that hot things start to glow. Increasing the temperature of a piece of steel changes its color from dark red, via cherry red, orange, yellow to white (and eventually blue if it wouldn't melt before that happens).
So by measuring precisely what kind of color (more precisely: spectrum) a hot piece of iron assumes, we can compute it's temperature.
What you probably don't know is that the composition of the light (the spectrum) that hot bodies emit is the same for all hot bodies. This includes your hot body, the sun, everything. If I would heat you up to 1.000 oC (1.832 oF), you would glow exactly like a piece of iron or whatever. I admit that heating you up that much in air would induce some changes in your body (described as "burning" or "oxidation") that are not enjoyable as we know from many experiments performed by the church a few hundred years ago. So we heat you up in an oxygen-free environment (not enjoyable either) and glow you will.
|It was Max Planck in 1900 who discovered the universal law of "black body" radiation (no, he wasn't thinking of your body after the experiment described above). His famous equations not only fits the observations very well and and allows us to measure temperature by just "looking" at an object with a spectrometer, it opened the door to quantum theory.|
|In fact, the glowing of hot objects
in "colors" that betray their temperature can only be understood by quantum theory. We have a
first little indication here that quantum theory is not just something
physicists amuse themselves with when dealing with exotic things that have
nothing to do with everyday life. Just the opposite.
Whenever a smith looks at his hot piece of steel to judge by its color if its temperature is right, he is doing quantum theory, probably without knowing that.
|We have two questions left. But I stop here. After all, this is a "BASIC" module.|
|I will, however, take up those questions at some other parts of the hyperscript. If you make it that far, you will learn that knowing the answer to question three is absolutely essential for understanding how to make good swords.|
|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
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
The Second Law
8.3.3 Bang it!
© H. Föll (Iron, Steel and Swords script)