A Temperature Scale

There are many quantities which vary with temperature

A hollow metal coil expands when heated and turns a needle	Some chemicals change colour with temperature	A thermocouple produces a p.d. proportional to temperature	Mercury in a capillary tube	Crickets rub their legs together faster when its hot

To make a thermometer we need a measurable quantity which varies with temperature. We will have to assume the relationship is linear, without another thermometer we have no way of testing whether it is linear or not.

We also need two fixed points upon which to base our scale. These are known temperatures that we can call 0 and 100. The classic ones are the freezing and boiling points of water at 1 atm pressure.

Different thermometers based on different quantities would agree with each other at the fixed points but would not necessarily agree at any other temperatures. This is because they would ALL assume a linear relationship.

What we need is a universal temperature scale that uses the same temperature dependant quantity. This is how the ideal gas scale or Kelvin scale came about.

The quantity it uses is the product of the pressure and volume of an ideal gas. So we assume that p V is proportional to T.

Thermometers based on the pressure or volume of gases seemed to agree quite well, also the fact that it is based on a mathematical model means it can never be disputed experimentally!

These are the fixed points used for the Kelvin Scale (or ideal gas scale)