Notes: Thermometric Properties

A thermometric property is one that varies with temperature. For example, most objects will expand as temperature increases. The volume of the object is a thermometric property.

It is from these properties that we get our temperature scales. Now, we can't just measure every degree between 1 and 100. The problem there is that different materials expand at different rates. An alcohol thermometer will not agree with a mercury one. So what you do is you get two points that are definitely what it says they are, then you just split everything in between evenly. Mostly the two points will be the temperature at which water freezes and boils at 1 atmosphere of pressure. Celsius for example used to be defined as such, putting freezing at 0 and boiling at 100 degrees, then defining a single degree Celsius as 1/100 of the boiling point..

These days things are a little more...precise? It's not changed too much but now Celsius handily ties into the Kelvin scale which has the fixed points as absolute zero and the triple point of water, the triple point being defined as the temperature at which a substance (in this case water) can exist as it's 3 phases (solid, liquid and gas) in thermal equilibrium. These points are given the numbers 0 K (-273.15 °C) and 273.16 K (0.01 °C), with a single Kelvin being defined as 1/273.16 of the temperature of the triple point of water.

Incidentally, avid QI fans like myself will remember this particularly contentious issue:


To be perfectly honest, I may not have actually gotten as far as writing in, but I was definitely bothered that he had gotten points for it the first time around.

I'm sorry Mal, I'm just bit sad like that.

Anyway, that was 2 lectures worth of notes. Christ, this particular module drags on a bit.