Instruments for Measuring Atmospheric Pressure

1. Simple Mercury Barometer

• made from a clean, dry and thick-walled glass tube about 100 cm long which is completely filled with mercury.
• the mercury column in the tube drops until its height is above the mercury level in the dish which is 760 mm Hg (standard atmospheric pressure or 1 atm) 
• the space between the mercury column in the tube is a vacuum known as  Torricellian vacuum.
• the height of the mercury column in the tube is unaffected by : 

          a) using glass tubes of different diameter.

          b) tilting the tube at different angles

2. Fortin Barometer

• its tube is enclosed in a metal case with glass windows at the upper part of the tube for viewing the mercury column.
• the height of the mercury column is measured from the tip of the ivory pointer to the mercury level in the tube.
• an accurate measurement of the height of the mercury column can be obtained by adjusting a vernier scale over the main scale.

3. Aneroid Barometer

• it contains no liquid and can be carried about conveniently
• consisits of a corrugated steel box which is sealed after some of the air from inside is pumped out.
• changes in the air pressure make the box contract or expand.
• the movements of the box are magnified by a system of levers which causes a pointer to move over a a calibrated scale.
• example : 

          a ) weather forecasting - low pressure indicates an approaching storm

                                             - high pressure is associated with fine weather.

          b) altimeters - located in aircrafts for measuring height