Wednesday, May 25, 2011

Measurement of Atmospheric Pressure


Earth is surrounded by a thick layer of air. Due to gravity of the earth, air is pulled towards it and exerts pressure on the surface of earth. The pressure exerted by the air layer is known as Atmospheric Pressure. The atmospheric pressure is measured by Barometer.
The barometer, in simple form can be made by taking a glass tube about 80 cm long. The glass tube is filled with mercury and then inverted in a vessel which also contains mercury. The level of mercury in the tube falls and then becomes static at certain height. At this stage, the pressure exerted by the column of mercury in tube is equal to pressure exerted by air (i.e. atmosphere pressure) on the mercury in vessel. The space above the mercury in the tube is perfect vacuum. Therefore, pressure due to mercury column is equal to atmospheric pressure. At sea level the height of column is observed to be 76 cm or 760 mm at 273 K.
CTM *(Mercury is generally used in barometer, because it has very high density and does not  stick to glass walls and is also not volatile at room temperature)

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