# How do mercury min/max garden thermometers work?

**URL:** <https://boards.straightdope.com/t/how-do-mercury-min-max-garden-thermometers-work/608029>\
**Category:** Factual Questions\
**Created:** [January 1, 2012, 11:27am UTC](https://boards.straightdope.com/t/how-do-mercury-min-max-garden-thermometers-work/608029 "2012-01-01T11:27:09Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\
**Post date:** [January 1, 2012, 11:27am UTC](https://boards.straightdope.com/t/how-do-mercury-min-max-garden-thermometers-work/608029/1 "2012-01-01T11:27:09Z")

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I tried Googling this to no avail…

There’s a particular design of garden thermometer that has two columns of mercury - sometimes apparently linked at the bottom (like a manometer) - each tube has a float that rests on top of the mercury - on the right hand side, this float stays in place at the maximum reach of the mercury, indicating the Max temperature.

But on the left side, it does the same - because the mercury column on this side apparently goes UP as the temperature decreases (the scale is marked in reverse on this side) so the float sticks at the minimum extent of temperature.

Except… how does it do that? How does the mercury column measuring minimum temeperature behave in the opposite manner to what should be expected?

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**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\
**Post date:** [January 1, 2012, 11:44am UTC](https://boards.straightdope.com/t/how-do-mercury-min-max-garden-thermometers-work/608029/2 "2012-01-01T11:44:13Z")

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Here is an [explanation of one](http://lib.store.yahoo.net/lib/yhst-63803600799585/5458troubleshooting.pdf)from it’s manufacturer. I’d seen one of these once before and had no idea what its purpose was. Apparently there is a clear liquid over the minimum side, which contracts and pulls the mercury up that side of the tube.