Measuring temperature

G. D. Fahrenheit, Anders Celsius and William Kelvin may sound like prospects for the Roughriders’ defensive backfield, but they probably wouldn’t work out very well. For one thing, they’re all dead. For another, they weren’t football players, but scientists–and two of them, at least, are household names, which is far less common for scientists than for football players.

What these three men have in common is that they all contributed a temperature scale to the world. This is a good way to get yourself remembered, because such scales are traditionally identified by their creators’ names.

Temperature is defined as the degree of hotness or coldness of a body (not that kind of body–a body in the sense of, say, a body of water, or a body of pistachio ice cream).

The hotter a substance gets, the faster its molecules move; as it cools, the molecules slow down again, so temperature is also defined as the average energy of the motion of the substance’s molecules.

It is possible to have extremely high temperatures that don’t translate to much actual heat. For example, in the outer fringes of the sun’s atmosphere the particles are moving very rapidly–so rapidly they have a temperature of millions of degrees–but they’re so widely spread that were we somehow transported to the scene, we’d feel no heat.

When we want to know the temperature of things here on Earth, we use a thermometer. There are various kinds, but most of them rely on there being a regular relationship between a change in temperature and a change, such as expansion and contraction, in some substance.

Galileo, in between discovering the moons of Jupiter and dropping things off the Leaning Tower of Pisa, invented one of the earliest thermometers. It was a glass bulb with a long, thin glass tube attached to it, inverted over a trough containing water in such a way that some of the water was sucked up the tube. When the air in the bulb was heated, it expanded and drove the water down the tube, the height of the water therefore providing a measure of the temperature of the bulb.

Only problem was, air thermometers are pretty inaccurate. A denser substance, such as mercury, would work better, thought Gabriel Daniel Fahrenheit, a German-born Dutch instrument maker. By the early 18th century, he was producing very accurate mercury thermometers; but obviously, in order for them to be of any use, they had to have some kind of scale marked on them. Fahrenheit chose to create a scale with 180 divisions between the freezing point of water, which he set at 32 degrees, and its boiling point, 212 degrees. Because his thermometers were so good and were widely used, the Fahrenheit temperature scale also became widely accepted.

Fahrenheit had been dead for six years when, in 1742, Swedish astronomer Anders Celsius proposed a new scale of temperature with a nice even 100 gradations between the freezing point of water, set at zero, and its boiling point. This “centigrade” scale also became widely accepted, though Celsius died just two years after he proposed it and therefore never got to hear his name repeated thousands of times a day in weather forecasts across Canada. (Of course, he might have found it a little hard to hear even if he had lived, since he’d be 290 this year . . . )

The Fahrenheit and Celsius scales share a common problem: they’re both completely arbitrary. There’s no compelling scientific reason to start a temperature scale at the freezing point of water.

What scientists really wanted was an absolute temperature scale. They got it from Lord William Thomson Kelvin. His temperature scale uses the same-size intervals as the Celsius scale (on the absolute scale the intervals are called Kelvins instead of degrees), but instead of starting at an arbitrary point, it starts at REAL zero–absolute zero–the temperature at which all molecular motion stops. Since temperature is determined by the motion of the molecules in a substance, once that motion stops, it’s impossible for the temperature to fall any lower.

Absolute zero, on the Celsius scale, is -273; or, to put it another way, water freezes at 273 Kelvin and boils at 373 K.

If you’re thinking that’s pretty darn cold, you’re right. At the Science Centre we do a show dealing with the science of cold temperatures, called cryogenics. We use liquid nitrogen, which has a boiling point of -196 C. A flower immersed in liquid nitrogen freezes solid and will shatter if you tap it on the table. Your fingers might do much the same. But on the absolute scale, liquid nitrogen is quite warm, having a temperature of 77 K–the difference between the coldest day of winter and the hottest day of summer.

That subtly segues us to the main interest most of us have in temperature, which is the temperature of the air. In Regina, the record high is 43 C, recorded on July 5, 1937. (The national record of 45 C was recorded in Midale that same day.) Regina’s record low is -50 C, eached on January 1, 1885. (The dubious honour of coldest spot in the nation goes to Snag, Yukon Territory, which reached -63 on February 3, 1947. Even that pales beside the world record of -73, recorded in Siberia.)

If a record high of 43 and a record low of -50 sounds like a pretty wide range of temperatures, you’re right; right here in southern Saskatchewan we have one of the most extreme climates on Earth, as far as the range of possible temperatures goes. It sounds even more extreme in Fahrenheit: 110 for a high and -58 for a low.

Fortunately, as I’ve shown, we have one other choice. We could start giving the temperature on the absolute scale. Then, even when our cars won’t start, our skin freezes in seconds, and plastics shatter like glass, we could bask in the knowledge that the temperature is still at least 223 above zero.

Doesn’t that sound better?

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