# Oral (and Rectal) Mercury Thermometers

**URL:** <https://boards.straightdope.com/t/oral-and-rectal-mercury-thermometers/192035>\
**Category:** Factual Questions\
**Created:** [July 31, 2003, 2:24am UTC](https://boards.straightdope.com/t/oral-and-rectal-mercury-thermometers/192035 "2003-07-31T02:24:08Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\
**Post date:** [July 31, 2003, 2:24am UTC](https://boards.straightdope.com/t/oral-and-rectal-mercury-thermometers/192035/1 "2003-07-31T02:24:08Z")

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Why is it necessary to shake down a mercury thermometer? I am puzzled why it holds the last reading instead of sinking back down to room (ambient air) temp. Even if the scale of the thermometer does not go this low, I would at least expect the mercury to contract until it is all collected in the bulb and off the scale.

Why is this not the case?

- Jinx

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**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [July 31, 2003, 2:42am UTC](https://boards.straightdope.com/t/oral-and-rectal-mercury-thermometers/192035/2 "2003-07-31T02:42:23Z")

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The capillary inside the thermometer has a “pinch” just above the bulb that restricts the flow from returning to the bulb when the temperature drops. You want a fever thermometer to hold the reading after it has been removed so the temperature can be accurately read.