Air in Mercury & Sprengel Pump Explained

[cragar]

I was reading about the sprengel pump , And it uses falling mercury to trap air to evacuate a chamber to get the air out of it , http://en.wikipedia.org/wiki/Sprengel_pump you can look at the diagram of the pump , my question is when the mercury traps the air and pushes it down , why doesn't the air just rise back to the top like air bubbles coming out of a divers mask back to the surface of the ocean.

 

[Borek]

It tries, but the diameter of the capillary tube is so small, and mercury surface tension so large, that air is trapped. It is just a matter of equilibrium of forces involved.

 

[cragar]

sweet thanks for the answer , is there any other fluid that i could use besides mercury so that i could make my own , so this has a lot to do with the coulomb force.

 

[Borek]

Will be difficult. Mercury is very good for two reasons - it has a very high density (hence can "suck" air much stronger than lighter liquids) and doesn't wet glass surface (which means it can't flow around the bubble on the glass walls).