The saturated vapour pressure of a liquid can be determined using the apparatus below. The air is first pumped out using a vacuum pump. The liquid is then boiled and evaporates. Some of the vapour fills the apparatus and some condenses. The water is then allowed to cool to a set temperature. At this temperature the water will exert a satuarted vapour pressure
If the atmospheric pressure is
and the liquid in the barometer column (mercury) has density
then
so
The air pressure within the apparatus can be varied using the cauum pump and the saturated vapour pressure of water can so be found for any air pressure.