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From: hes@ecsvax.UUCP (Henry Schaffer)
Newsgroups: net.puzzle
Subject: Re: Thermometer Puzzle  **SPOILER**
Message-ID: <633@ecsvax.UUCP>
Date: Fri, 25-Oct-85 21:50:52 EDT
Article-I.D.: ecsvax.633
Posted: Fri Oct 25 21:50:52 1985
Date-Received: Sat, 26-Oct-85 19:45:21 EDT
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Organization: NC State Univ.
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> When an ordinary mercury thermometer at room temperature is inserted
> into a beaker of boiling water, the mercury actually descends a
> small but noticeable amount before ascending to the 100 C level.
> Why?
> 
> Steve Harris  
 
    Before the mercury can expand because of the heat, the glass
 surrounding the mercury must get hot to pass on the heat.  The glass
 does expand somewhat *before* the mercury heats up.  This expansion
 increases the volume of the "container" holding the mercury, and so
 the mercury will descend.  For a small (rate of) change in the temperature 
 the effect should be very small, but room temp. to boiling water is not
 a small change.
 --henry schaffer