Civil Engineering Questions
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Ali Hamza
Ali Hamza
25 Jan 2019

What is Barometer??


If a tube lower end is immersed in a liquid which is exposed to the atmospheric pressure, andif air is exhausted from the tube, the liquid will rise in it. If air is completely exhausted, theonly pressure on the surface of the liquid in the tube would then be that of its own vaporpressure and the liquid would have reached its maximum height.The pressure at O within the tube and at a at the surface of liquid outside the tube must be the same; that is Po = Pa

Patm A – Pvapor A – 𝜸Ay = 0

Patm = 𝜸y +Pvapor

If vapor pressure on the surface of liquid in the tube were 

negligible then 

Patm = 𝜸y 

The liquid employed for barometers is usually mercury because its density is sufficiently greatto enable a reasonably short tube and its vapor pressure is negligible at ordinary temperature.Corrections of capillarity and vapor pressure should be applied.