Atmosphere is a gaseous envelope surrounding the earth and the pressure exerted by the atmosphere is called atmospheric pressure.
The cause of atmospheric pressure is motion of air molecules which are in continuous mtion.
The molecules strike the surface of body placed in it and exert a huge force.
To measure the atmospheric pressure,
i) Torricelli took a meter long graduated tube and filled it with clean and dry mercury.
ii) By closing the tube with thumb, he inverted the tube in a cistern ( a tub filled with mercury) as shown in the figure.
iii) He observed that the level of mercury first fell down and finally stayed with a column of height 76cm in the tube above the free surface of mercury in the cistern leaving behind vacuum.
iv) At point C, vacuum is formed. Therefore, pressure at this point is zero. Point B in the tube is 76cm below C. So, pressure at B is given by,
PB =PC + ρgh = 0 + ρgh = ρgh
where ρ is density of mercury and h be the height column of mercury in the tube above point B.
In the given figure, point A is at the interface of air and mercury, therefore it is both in air and as well as in mercury.
Thus pressure at point A is equal to atmospheric pressure, i.e.
PA = Atmospheric pressure as vertical height between A and B is zero, therefore pressure at A and B is same, i.e.
Atmospheric pressure = pgh
= 13600 x 9.8 x 0.76
= 1.013 x 105 N/m2