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This question already has an answer here:

In barometer the height of mercury in the inverted tube above the open surface level will be 76cm, if mercury is used. Now what if we take a tube filled with mercury but of length less than 76cm and invert it into the tub? (Experiment being done at the sea level)

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marked as duplicate by Kyle Kanos, Ben51, John Rennie, Jon Custer, user191954 Nov 30 '18 at 6:55

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ The mercury will rise to the very top of the tube. Then, if you measure it's hight to get the pressure, your measurement will be invalid, as the mercury cannot rise as high as it wants to. $\endgroup$ – hdhondt Nov 29 '18 at 9:38
  • $\begingroup$ @hdhondt, what happens to the tube after the mercury raises towards the top of the tube $\endgroup$ – sheshin Nov 29 '18 at 10:48
  • $\begingroup$ @hdhondt Your first comment should be posted as an answer. $\endgroup$ – safesphere Nov 29 '18 at 15:00
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Although the question already has an answer, as pointed out by @Farcher and @Kyle Kanos, I'll repeat my comment here.

The mercury will rise to the very top of the tube. Then, if you measure it's height to get the pressure, your measurement will be invalid, as the mercury cannot rise as high as it wants to.

As long as the tube is strong enough, the glass will exert a downward pressure on the mercury, which will counteract the excess pressure from the atmosphere. If the glass is not strong enough, then the pressure will shatter it, after which the mercury will drop back into the bath.

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