Oral (and Rectal) Mercury Thermometers

Factual Questions
1

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
2

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.