Instruments for Measuring Atmospheric Pressure

A) Simple Mercury Barometer

Simple mercury barometer

1) It is completely filled with mercury. Any air bubbles inside are removed by inverting the tube several times and filling it again with mercury.
2) The tube is inverted into a dish of mercury with its open end well below the mercury level.
3) The space above the mercury column in the tube is a vacuum known as Torricellian vacuum.

B) Fortin Barometer 

Fortin Barometer

1) A Fortin barometer is an improved version of a simple mercury barometer. Its tube is enclosed in a metal case with glass windows at the upper part of the tube for viewing the mercury column.

C) Aneroid Barometer

Aneroid Barometer

1) An aneroid barometer is used more often than a mercury barometer because it contains on liquid and can be carried about conveniently.

2) Change in 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 calibrated scale.