# Why doesn't the mercury in thermometers shrink in response to low temperatures? **URL:** \ **Category:** Factual Questions\ **Created:** [May 16, 2022, 2:45pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499 "2022-05-16T14:45:56Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![SpacemanSpiff\_II](https://avatars.discourse-cdn.com/v4/letter/s/e19adc/32.png) [@SpacemanSpiff\_II](https://boards.straightdope.com/u/SpacemanSpiff_II)\ **Post date:** [May 16, 2022, 2:45pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/1 "2022-05-16T14:45:56Z")
Why is it that these thermometers will measure temperature by having the mercury (or some other substance used in some of the newer ones) expand in response to higher temperatures, but it does not shrink in response to lower temperatures, and you need to shake it to get it to go down? ---
**Author:** ![3AxisCtrl](https://avatars.discourse-cdn.com/v4/letter/3/ecd19e/32.png) [@3AxisCtrl](https://boards.straightdope.com/u/3AxisCtrl)\ **Post date:** [May 16, 2022, 2:52pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/2 "2022-05-16T14:52:31Z")
You’re describing a special thermometer, designed to read and hold the maximum temperature sensed. Here’s the [relevant Wiki paragraph](https://en.wikipedia.org/wiki/Mercury-in-glass_thermometer): > One special kind of mercury-in-glass thermometer, called a maximum thermometer, works by having a constriction in the neck close to the bulb. As the temperature rises, the mercury is pushed up through the constriction by the force of expansion. When the temperature falls, the column of mercury breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. The observer can then read the maximum temperature over the set period of time. To reset the thermometer it must be swung sharply. ---
**Author:** ![SpacemanSpiff\_II](https://avatars.discourse-cdn.com/v4/letter/s/e19adc/32.png) [@SpacemanSpiff\_II](https://boards.straightdope.com/u/SpacemanSpiff_II)\ **Post date:** [May 16, 2022, 5:08pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/3 "2022-05-16T17:08:04Z")
Answered in one post! Thank you very much. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [May 16, 2022, 5:26pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/4 "2022-05-16T17:26:10Z")
You must be young. This is the way all “fever” thermometers worked up to the 70’s or maybe 80’s. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [May 16, 2022, 6:26pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/5 "2022-05-16T18:26:18Z")
> [@beowulff](#): > > This is the way all “fever” thermometers worked up to the 70’s or maybe 80’s. That’s a really low grade fever. ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [May 16, 2022, 6:31pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/6 "2022-05-16T18:31:29Z")
> [@Riemann](#): > > > [@beowulff](#): > > > > This is the way all “fever” thermometers worked up to the 70’s or maybe 80’s. > > That’s a really low grade fever. - Not in Celsius. 🥵 ---
**Author:** ![EdwardFound](https://avatars.discourse-cdn.com/v4/letter/e/4af34b/32.png) [@EdwardFound](https://boards.straightdope.com/u/EdwardFound)\ **Post date:** [May 16, 2022, 7:06pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/7 "2022-05-16T19:06:20Z")
Not necessary to young; I remember those thermometers very well, but I never realized why the mercury didn’t return to the bulb. I believe an early draft of the Outkast lyric was, “Shake it like a mercury thermometer.” ---
**Author:** ![Honolulu\_Lulu](https://avatars.discourse-cdn.com/v4/letter/h/87869e/32.png) [@Honolulu\_Lulu](https://boards.straightdope.com/u/Honolulu_Lulu)\ **Post date:** [May 16, 2022, 8:03pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/8 "2022-05-16T20:03:04Z")
> [@SpacemanSpiff\_II](#): > > but it does not shrink in response to lower temperatures, Yes, it does shrink. It just doesn’t go down until you shake it. If it didn’t shrink it would not go down even after shaking. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [May 17, 2022, 3:01pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/9 "2022-05-17T15:01:32Z")
> [@beowulff](#): > > You must be young. > This is the way all “fever” thermometers worked up to the 70’s or maybe 80’s. Uh… they don’t anymore? How do they work now? Not sure, but I might not have replaced my fever thermometers since then… ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [May 17, 2022, 3:03pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/10 "2022-05-17T15:03:54Z")
> [@Napier](#): > > How do they work now? Same but with electrons squeezing through tiny tubes drilled in silicon? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [May 17, 2022, 3:42pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/11 "2022-05-17T15:42:01Z")
Mercury fever thermometers are being phased out: > **[Mercury Thermometers | US EPA](https://www.epa.gov/mercury/mercury-thermometers)** > > Learn about mercury thermometers, their uses, the laws regulating their sale, alternatives to them, and how to cleanup a broken mercury thermometer Modern thermometers are electronic, and more accurate. ---
**Author:** ![SpacemanSpiff\_II](https://avatars.discourse-cdn.com/v4/letter/s/e19adc/32.png) [@SpacemanSpiff\_II](https://boards.straightdope.com/u/SpacemanSpiff_II)\ **Post date:** [May 17, 2022, 3:54pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/12 "2022-05-17T15:54:24Z")
> [@beowulff](#): > > Mercury fever thermometers are being phased out: > > Modern thermometers are electronic, and more accurate. Those are not the only two choices. We just got a new thermometer that works like a mercury thermometer but uses some substance other than mercury. [Geratherm Thermometer, Oral, Mercury-Free, 1 thermometer | Rite Aid](https://www.riteaid.com/shop/geratherm-thermometer-oral-mercury-free-1-thermometer-0324122) We actually have a working digital thermeter, but - contrary to your claim - they are much less accurate than the old-fashioned type (though they are a lot faster), and I finally got sick and tired of them and went back to the old style. ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [May 17, 2022, 4:02pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/13 "2022-05-17T16:02:06Z")
> [@beowulff](#): > > Mercury fever thermometers are being phased out: Last year I tried to order a mercury equine thermometer and was surprised I couldn’t find one online. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [May 17, 2022, 5:41pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/14 "2022-05-17T17:41:39Z")
Used _correctly_, a digital oral or tympanic thermometer is going to be more accurate than a mercury thermometer, and a lot faster. But an IR non-contact thermometer is likely to be wildly inaccurate. ---
**Author:** ![DavidNRockies](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/davidnrockies/32/6279_2.png) [@DavidNRockies](https://boards.straightdope.com/u/DavidNRockies)\ **Post date:** [May 17, 2022, 5:44pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/15 "2022-05-17T17:44:23Z")
“A fever thermometer is solitary, poor, nasty, brutish, and short.” – Kelvin and Hobbes ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [May 17, 2022, 6:14pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/16 "2022-05-17T18:14:11Z")
> [@beowulff](#): > > Used _correctly_ , a digital oral or tympanic thermometer is going to be more accurate than a mercury thermometer, and a lot faster. But an IR non-contact thermometer is likely to be wildly inaccurate. a tympanic thermometer _is_ a non-contact IR thermometer: it doesn’t actually touch your eardrum. It’s more accurate than a forehead thermometer because your forehead is subject to all kinds of environmental effects, whereas your eardrum is protected from wind, sun, and evaporative cooling effects by being located deep in a cave, accessible only via narrow tunnel. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [May 17, 2022, 6:39pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/17 "2022-05-17T18:39:16Z")
> [@SpacemanSpiff\_II](#): > > We just got a new thermometer that works like a mercury thermometer but uses some substance other than mercury. Probably uses galinstan, an alloy of gallium and indium and tin (stannum). Marvelous alloy, but it wets everything, so these are coated on the inside to prevent that. Indium oxide, iirc. ---
**Author:** ![SpacemanSpiff\_II](https://avatars.discourse-cdn.com/v4/letter/s/e19adc/32.png) [@SpacemanSpiff\_II](https://boards.straightdope.com/u/SpacemanSpiff_II)\ **Post date:** [May 17, 2022, 7:01pm UTC](https://boards.straightdope.com/t/why-doesnt-the-mercury-in-thermometers-shrink-in-response-to-low-temperatures/964499/18 "2022-05-17T19:01:24Z")
> [@beowulff](#): > > Used _correctly_ , a digital oral or tympanic thermometer is going to be more accurate than a mercury thermometer, and a lot faster. The version we have at home is some version of this: [https://www.amazon.com/Thermometer-Electronic-Underarm-Readings-Children/dp/B0007RTEAI?th=1](https://www.amazon.com/Thermometer-Electronic-Underarm-Readings-Children/dp/B0007RTEAI?th=1)). It’s a digital oral. Tends to read low. > [@beowulff](#): > > But an IR non-contact thermometer is likely to be wildly inaccurate. The ones used by medical professionals tend to be non-contact. I’ve spoken to some of them about it and they agree that they’re inaccurate. The thing is that the temperature in a medical setting is only a small part of what they’re looking at, so a rough approximation is OK. Conversely, time is a big deal. (Not to mention covid temperature-checks, which would be completely impractical with mercury-type thermometers.) But for a lay person at home, who is looking at temperature to distinguish between “woke up on the wrong side of the bed” and “genuinely ill”, accuracy is a bigger deal and time is a smaller one.