Heat ( Be quick ! )

Celsius proposed a method of calibrating a liquid-in-glass thermometer:

1. Place the cylinder of the thermometer in melting pure water and mark the point where the fluid in the thermometer stabilises. This point is the freeze/thaw point of water. (0 C)
2. In the same manner mark the point where the fluid stabilises when the thermometer is placed in boiling water vapour. (100 C)
3. Divide the length between the two marks into 100 equal pieces.

So the two thermometers will read the same temperature at 0C and 100C. However, the coefficients of expansion of the two substances may not be perfectly constant. They may vary in different rate as the temperature changes. So, the two thermometers may not show the same reading at other temperatures.