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B Mercury and thermometer

  1. Jan 5, 2017 #1
    Why does mercury in a thermometer expands when we increase the temperature but it doesnt go down after cooling? Why does it just stay there?
     
  2. jcsd
  3. Jan 5, 2017 #2

    Drakkith

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    As far as I know it does contract. If it didn't, it would only show the highest temperature reached and would be of little to no use in a thermometer.
     
  4. Jan 5, 2017 #3
    You can only shake the mercury off when you need to measure the temperature again. The time it contracts is very long i suppose since after a week it will still be roughly the same.
     
  5. Jan 5, 2017 #4
    I am still looking for the answer to the question : " Mercury expands in a thermometer when temperature is in increased, why doesnt it reduce in size when its been in a lower temperature for a long time?"
     
  6. Jan 5, 2017 #5

    Nidum

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    In a normal Mercury thermometer the mercury level changes rapidly with the bulb temperature .

    I would guess that the time constant is of order of a few seconds for a small thermometer .

    There is a type of mercury thermometer that is arranged to record the highest temperature reached . They have a constriction in the neck which slows the fall of the mercury as bulb temperature drops . Some clinical thermometers used to have this feature . Vigorous shaking helped the mercury to fall more quickly .
     
  7. Jan 5, 2017 #6
    Can you explain what constriction?
     
  8. Jan 5, 2017 #7

    Nidum

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  9. Jan 5, 2017 #8

    Drakkith

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    I just saw this in wikipedia's article on mercury thermometers before coming back to this thread! :biggrin:
     
  10. Jan 5, 2017 #9
    Why does it detach?
     
  11. Jan 5, 2017 #10

    sophiecentaur

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    The narrow constriction in a 'Medical Thermometer' allows the column to break because the effect of cohesion of the mercury must be greater than the adhesion to the glass and over the tiny area of the constriction, the top and bottom regions of mercury pull together and away from the nearby glass. I imagine that the space inside the tube will be at low pressure so the two bits of the column will not be pushed together by atmospheric pressure.
    You will notice that only a slight tap or shake is needed to bring the upper part of the column back down.
    I wouldn't mind betting that the 'stay up' feature came about by an accidental poor bit of glass drawing and a small kink was identified as what was wrong - but it turned out to be a lucky mistake.
    There is an alternative system (used in old fashioned max/min room thermometers) which uses a tiny split pin of steel. The mercury pushes the pin up and it stays there as the mercury retreats. You bring it back down with a magnet.
     
  12. Jan 8, 2017 #11
    What I want to know more is about the normal thermometer. The bulb mercury detaches from the column mercury and my guess is that its because of surface tension or interaction between molecules, whether it would be mercury mol and glass mol or mercury mol and mercury mol. The problem is that i cant explain it, can you help?
     
  13. Jan 9, 2017 #12

    sophiecentaur

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    You probably read the term "medical thermometer" and didn't go much further with my post but exactly the same thing is at work in regular 'liquid in glass' thermometers. The adhesion of mercury to glass is much lower than the cohesion of mercury to itself; that's the comparison between the attraction the molecules have for each other. You can tell this when you look at the meniscus of the top of a mercury in glass column. It is highly convex, which shows the mercury is pulling itself together rather than wetting the glass - which is the opposite of what you get with water against the sides of a glass container (concave meniscus).
    Why does it happen in a thermometer? To make a thermometer as sensitive as possible, you need a narrow bore so that the change in the column length is as great as practical for the size of the reservoir in the bulb. It is possible that the thermometer you are using is fairly sensitive (distance between 1° marks) and that will involve a thin bore. Any small disturbance can cause the column to split up in the above way. This 'flaw' is used to advantage in a clinical thermometer as it retains the highest temperature measured.
    The other possibility is that your thermometer is not very good and may have small impurities along the column, where the mercury can separate. I have not actually come across this happening with a thermometer that's treated with 'respect' but it's always a potential problem.
    They banned mercury for school thermometers years ago and the alternative alcohol thermometers (red column) are also susceptible to physical abuse. The lab techs often spend their time, after lessons, tap tapping the cheapo thermometers that have been dropped and bashed by the kids, to bring the sections of the alcohol columns back together.
     
  14. Jan 9, 2017 #13
    Thanks for the answer. Im sorry that I didnt understand the first time. Its just because im not well taught in the scientific language. I guess ill have to translate it to latvian. Thanks again really, this might earn me a 10/10 in physics.
     
  15. Jan 9, 2017 #14
    Failed again, sheesh
     
  16. Jan 9, 2017 #15

    Drakkith

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    What do you mean?
     
  17. Jan 10, 2017 #16
    I wanted to quote sophiecentaur but i quoted twice.
     
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