A mercury barometer and how it works

A mercury barometer consists of a glass tube which is inverted over a bowl of mercury. The glass tube is marked in mm and it’s illustrated below

• Air has weight and therefore exerts pressure on the Earth’s surface; it pushes the surface of  mercury and cause mercury to rise up in a glass tube.
• When atmospheric pressure falls, the force on the mercury surface reduced and mercury is forced to flow out of the glass tube leading the height of mercury column to fall.
• The higher the length of mercury column in glass tube, the higher the atmospheric pressure.
• It is expressed in millibars where 750mm of mercury corresponds to 1000 millibars (mbs)