| Measuring
air pressure : Torecelli was the first to realize that air pressure can
be measured by height of liquid columns that the pressure can balance and
sustain. He took a glass tube
more than 10 m in length, filled it with water and inverted it in a tub
filled with water. The water level fell in the tube till such time that the
pressure of the water column at its base was same as the atmospheric
pressure on the surface of the water in the tub. The height of the water in
the tube was 10.34 m. Barometers
are used to measure atmospheric pressure. Instead of water, mercury (Hg) is
used as a preferred liquid as the density of Hg is 13.6 gm/cm3,
hence the height of the liquid column balancing the atmospheric pressure
decreases considerably. This we shall see in the next section. |
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Mercury
Barometer : Take a hard
glass tube about 1 m long, closed at one end and with a narrow bore. Fill it
up completely with mercury. Hold
a finger on the mouth of the tube and invert it in a dish containing
mercury. The column of Hg in the tube will drop a little. There will be a
bit of vacuum at the end of the tube. This is called Toricelli vacuum.
Let us now see how the column of Hg is a measure of atmospheric
pressure.
Consider a point A on the
surface of Hg in the dish. Consider another point B at the same level but
inside the tube. A and B have to be at the same pressure. (Liquid seeks its
own level till the pressure is equalized).
Pressure at A =
atmospheric pressure
Pressure at B = height of
the Hg column x  Hg
x g
Thus atmospheric pressure
= height of the Hg column x Hg
x g
It is observed that the
height of the Hg column at sea level is 760 mm of Hg.
This is the standard measure of pressure. Thus 1 atmosphere = 760 mm
of Hg.
The use of Hg as a
suitable liquid in pressure measuring barometers has many advantages:
-
The height of the
liquid Hg column is manageable because its density is high (13.6 gm/cm3).
-
Atoms of Hg have high
cohesive forces in them. Hence Hg atoms do not stick to glass tube. Any
change in pressure thus is adjusted quickly by the Hg column.
-
Hg has a very low
freezing point (-30ƒC)
and hence such a barometer can be used at places where the atmospheric
temperature is low as in high altitudes.
-
Hg is a shining
liquid. Reading off the Hg column height from the barometer is easy.
As
the atmospheric pressure changes, the height of the Hg column will change.
It should be noted that the height of the Hg column does not depend on the
shape or the size of the tube. The height does not change even if the
column is titled.
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