Describe mercury barometer for measurement of atmospheric pressure.
OR Describe Torricelli experiment for measurement of atmospheric pressure.

Explanation

Italian scientist Evangelista Torricelli devised for the first time ,the method for measuring atmospheric pressure.
This device is shown in figure

A long glass tube of 1 m closed at one end and filled with mercury is inverted into a trough of mercury . Thumb is kept on open part of tube.
If thumb is taken from tube ,level of mercury column slightly decreases.
The space above the mercury column in the tube contains only mercury column in the tube contains only mercury vapour whose pressure P is so small that it may be neglected . P=0.
The Pressure inside the column at point A = the pressure at point B, which is at the same level. Atmospheric pressure at point A
`P_(a)=P+hrhog`
`P_(a)=O+hrhog`
`thereforeP_(a)=hrhog` ... (1)
Where `rho=` density of mercury ,h= height of the mercury column.
In this device height of mercury column at sea level is 79 cm which is equivalent to one atmosphere.
`h=76cm=0.76m`
`rho=13.6xx10^(3)kg//m^(3)`
`g=9.8m//s^(2)`
Atmospheric pressure
`P_(a)=(0.76)(13.6xx10^(3))(9.8)`
`P_(a)=1.013xx10^(5)` Pascal
`=1.013xx10^(5)N//m^(2)`
In common way of stating pressure is in terms of cm or mm of mercury Hg.