To define the Celsius temperature scale two "reliable" temperatures are used. These are, the melting point of pure ice, which gives us the lower fixed point of the scale and the boiling point of pure water (at atmospheric pressure) which gives us the upper fixed point. The difference between these two points is divided into 100 equal intervals called degrees.

Definition of the lower fixed point (called 0°C).	Definition of the upper fixed point (called 100°C).

It should be noted that this is a purely arbitrary scale. We could have called the temperature of melting ice -28°C or 13°C and the temperature of boiling water 115°C or 2500°C, but why complicate things?! Similarly we could have chosen another substance instead of water.

The Kelvin or Absolute scale of temperature is a more fundamental scale. (This scale is discussed in more detail in the section "The Pressure Law and the Absolute Zero of Temperature".)

The size of the degree on the Kelvin scale is the same as on the Celsius scale but the zero corresponds to a very different temperature. To convert temperatures on the Celsius scale to temperatures on the Kelvin scale simply add 273 to the Celsius reading.

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