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Question
#24888. Tom
asks:
A medical thermometer has a constriction in the tube that prevents the mercury from falling as soon as you remove the thermomerter from your mouth. However, the mercury did go once through constriction when the temperature was being taken. Why won't it go back?
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sequoianoir
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Doesn't just apply to a medical thermometer but any 'maximum' thermometer that uses the contriction princial.
As the air temperature rises during the day, the mercury in the thermometer expands and moves freely up the tube past the constriction. When the air temperature begins to drop, the constriction prevents the mercury from flowing back down the tube.
The pressure at the constriction is much higher than the pressure in the rest of the tube. In order for the mercury to move into the bulb, it must overcome the pressure in the constriction. The free-flowing mercury does not have enough pressure to do this by itself and thus requires an outside force (your hand shaking the thermometer) to overcome the constriction.
 http://www.windows.ucar.edu/earth/Atmosphere/temperature/max_therm.html
Dec 06 02, 2:22 AM
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