# 3 Temperature Scales

There are 3 temperature scales in wide use today, these are Fahrenheit, Celcius, and Kelvin. A variety of other scales are in use but have specialized uses in engineering.

Before discussing temperature scales we should study where the idea of heat and temperature came from and how temperature was originally measured. We’ll study modern methods of temperature measurement in another lesson.

In ancient times (ie prior to 1650) philosophers would debate whether it was heat that flowed or was it cold that flowed?

To some extent this idea is still alive in our modern mythology

Buy about the 1600’s the popular belief was that heat was a magical fluid called “Caloric” which flowed from one object to another.

Caloric, it was believed,  could be transferred between objects but neither created nor destroyed. To heat up an object this caloric had to flow into it. This, they thought, explained why objects expanded when heated. But this theory could not explain, for example, how heat could emanate from a cold piece of wood once it is set on fire? Where did the caloric come from? If it had been in the wood in the first place, the wood should have been hot all along.
The caloric theory was abandoned in the 19th century and replaced with the kinetic-molecular theory. This new theory stated that all matter is made up of atoms/molecules in constant motion. The faster they move, the hotter an object will be.

What is temperature?

We learned earlier that the internal energy of a system is the sum of the individual kinetic energies of all the system’s parts (atoms, molecules). Temperature is the AVERAGE value of kinetic energy of all the parts.Think of temperature as being the internal energy divided by the number of parts in the system.