My question is quite straight forward: What forces are pushing mercury up a capillary tube and then preventing it from going higher as tube gets thinner?

Setup: We've got a container filled with mercury and a capillary tube placed in it

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    $\begingroup$ Mercury doesn't go up in a capillary tube. It goes down. $\endgroup$ – Whiskeyjack Jul 27 '18 at 6:50
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    $\begingroup$ Could you describe the setup you have in mind? $\endgroup$ – V.F. Jul 27 '18 at 11:13

As the mercury changes temperature, it expands or contracts at a much greater rate then it's container. If the temperature gets hotter the mercury expands, the force exerted by the mercury on the capillary walls increases. Since there is an opening above, the mercury expands into this opening. It stops rising when the force exerted by capillary walls equals the force due to gravity pulling the liquid back down. The thinning of the capillary is used to increase the accuracy of the thermometer. As the capillary thins, the mercury moves more for a given temperature change.

You can look at the problem as being due to the increase in volume due to the expansion of the mercury. If the mercury was in a sealed container the pressure would increase. In the case of the thermometer, the increase in volume causes the mercury to expand into the capillary. The amount it rises in the tube is equal to the increase in volume the mercury experiences.

  • $\begingroup$ (Thank you for answering)What do you mean by "force exerted by capillary walls"? Does capillary exert any force ?If it's true, how? $\endgroup$ – Abbas Jul 27 '18 at 17:37
  • $\begingroup$ And also , doesn't it have anything to do with air pressure? $\endgroup$ – Abbas Jul 27 '18 at 17:38
  • $\begingroup$ Not if your capillary is sealed. $\endgroup$ – jmh Jul 27 '18 at 17:59
  • $\begingroup$ Was your comment on first one or second one? $\endgroup$ – Abbas Jul 27 '18 at 18:26
  • $\begingroup$ If mercury exerts a force on capillary walls, the capillary walls exert an equal but opposite force on the mercury. $\endgroup$ – jmh Jul 27 '18 at 18:54

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