How does the mercury in a barometer stay suspended?

Question

Is it the vaccum at the top of the tube? The air pushing on the mercury outside the tube? Something else?

Answer 1

In a barometer which is of the mercury type,the weight of the column of mercury is made to balance the pressure of the atmosphere over a mercury trough.
As you said ,the outside air pushes the mercury in the cistern(trough) to rise in the tube (which is above the cistern)to a height which depends upon the atmospheric pressure.At the top of the tube ,vaccum should be there.Otherwise ,the pressure of air inside will oppose the rise of the mercury column, if the outside atmospheric pressure increases and the value shown will not be correct.

Answer 2

The air trapped in the tube is at a constant pressure, depending on the air pressure outside the tube the air in the tube expands or contracts casing the level of the mercury to rise or fall.

Answer 3

I agree with Brian answer . Mercury must be sensitive to the fluctuations in the air inside the tube and gives reading according to the differences in the air inside the tube .

Answer 4

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