Lecture 5: Pressure variation in two immiscible fluids, manometer, barometer
(3) Barometer (Instrument to measure atmospheric pressure)
(Fig. 5b)
Neglect surface tension or capillary effects
Height of the liquid in the vertical tube is h measured from the free surface.
On integration,
But, at the same level
and
Therefore, . is nothing but the vapor pressure of Hg in the space over
the liquid Hg in the tube. Generally, (very small at Room temperature). Hence,
. One can experimentally measure to show that
There are two problems in using water-barometer
Tube is long!
Vapor pressure of water is significantly larger than that of Hg, i.e.,
and should be considered in the calculation!!
and 
. 
is nothing but the vapor pressure of Hg in the space over
the liquid Hg in the tube. Generally, 
(very small at Room temperature). Hence,
. One can experimentally measure to show that
and should be considered in the calculation!!
