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Mercury barometer

Mercury barometer 



This is the most common type of barometer used.  Principle: Mercury is a liquid which has high density (13.6 g / cc).  Hence, it can rise as a short column when the pressure exactly balances the atmospheric pressure.  Moreover, mercury does not stick to the walls of the glass tube due to weak adhesive forces between them.  

Construction and working
A critical glass tube of about 840-mm length which is closed at one end and open at the other end is taken.  This tube is filled completely with mercury and then inverted into a trough hil mercury.  Depending on the external pressure, some mercury from glass tube comes out and stae at a certain height.  This height is measured.  Above this column, there is almost vacuum and called torricellian vacuum.  

At normal atmospheric pressure at sea level, the height of the mercury column falls to a00 mm of Hg.  Hence, this is taken as the standard value of atmospheric pressure.  1 atm 760 mm of Hg = 760 torr 101.3 kilopascals The pressure can be calculated by substituting the height of the mercury column in the forma P hdg, where 'h' is the height of the mercury column, 'd' is the density of the  mercury and s the acceleration due to gravity.  Aneroid barome