I quickly plunged a room-temperature thermometer into very hot water, the mercury level went down briefly before going up to a final reading. Why?

  • $\begingroup$ Any chance you can link to a video? $\endgroup$ – BMS Aug 20 '14 at 16:12
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    $\begingroup$ Maybe the glass cylinder expanded slightly due to the increase in temperature? $\endgroup$ – jkej Aug 20 '14 at 16:14
  • $\begingroup$ I have noticed this before as well. $\endgroup$ – NeutronStar Aug 20 '14 at 16:34
  • $\begingroup$ @BMS:I just noticed it practically... i am trying to search for a video for your convenience... $\endgroup$ – Shamayeta Aug 21 '14 at 11:12

As it was done quickly, the mass of the mercury remained at room temperature for a short period of time, even as the thermometer itself (including the metal bulb) began to heat.

I suppose it is possible that even though it is a tiny portion, the metal bulb expanded sufficiently so that the volume inside the bulb increased and the level of the mercury went down.

Shortly after the mercury itself heated and you were able to see the expansion.

  • $\begingroup$ This also suggests an interesting issue: A thermometer should read differently when only the bulb is submerged rather than the entire thermometer. At first because T_HgBulb > T_Hgcol then because of thermal expansion of the glass of the column (cte of glass ~ 1/10th that of Hg). Somebody probably looked at this 'back in the day'. Now we deal with conduction losses along our thermocouple/rtd leads. $\endgroup$ – user3823992 Aug 22 '14 at 8:42

Instead of the expansion of the bulb which should be slight, it might also be the air in the thermometer.

According to wikipedia, it is filled with nitrogen or air at lower than 1 atm.

When we compare this small mass of gas with the "large" mass of mercury, it will most likely heat up faster even with regards to heat capacities.

At room temperature, most household thermometers will have more air volume than mercury volume. This also means a larger surface area for conduction then convection.

As the air heats up, it will expand, pushing the slow-heating mercury down. The mercury will push back as it slowly heats up.

Pretty much, it is a race, and the air is faster.

  • $\begingroup$ I don't think mercury is compressible, and therefore, that the expansion of air has any effect. The pressure of air will increase, but that shouldn't affect the mercury. $\endgroup$ – Émile Jetzer Sep 16 '14 at 13:53
  • $\begingroup$ Well mercury is compressible up to a certain point. I just assumed the "went down briefly" was just pushing down the meniscus and mercury a little bit. If it showed a large temperature difference, then it would not be this small compression. $\endgroup$ – flewk Sep 16 '14 at 15:48
  • $\begingroup$ Mercury is about as compressible as water, or any Newtonian fluid: not enough to see a difference near room temperature and pressure. Mercury is also opaque, so we cannot see the meniscus' shape. If the center of the meniscus is lowered by an increase in pressure, what we'll see is a raise in the visible mercury level. $\endgroup$ – Émile Jetzer Sep 17 '14 at 16:34

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