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A barometer is kept inside a bell jar. Air is slowly pumped out of the jar.

If this experiment is performed my book says that the mercury rises. But why so? The barometer is based on the principle of pressure exerted by the air on its open surface. If there's no air it should fall isn't so?

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    $\begingroup$ Consider including the name of the book and/or a figure. $\endgroup$ – Puk Jul 25 at 6:34
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    $\begingroup$ Yeah you could include all quotes and figures, and yes books have typos $\endgroup$ – user47014 Jul 25 at 6:37
  • $\begingroup$ It's in my native language and a native writer would it help if I mention it? I don't think nobody even heard about him. $\endgroup$ – Ritwik Bhattacharyya Jul 25 at 22:41
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The mercury 'wants' to rise, but is pressed down by the weight of the atmosphere at one end and held back by the partial vacuum at the other. When the air is pumped out of the jar, there is less pressure on the open end of the mercury column to keep it down, so it rises. If the pressure were reduced so that there were a vacuum at both ends of the column, or alternatively, the same air pressure at both ends, the mercury would find its own level just as water does.

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  • $\begingroup$ I think rising in a barometer usually refers to the closed end $\endgroup$ – user47014 Jul 25 at 9:49

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