MadSci Network: Physics

Re: Use of Constriction in Clinical Thermometer.

Date: Tue May 5 00:40:11 1998
Posted By: Don Pettibone, Other (pls. specify below), Ph.D. in Applied Physics, Quadlux Inc.
Area of science: Physics
ID: 891268945.Ph

There is a wonderful book entitled “The Flying Circus of Physics”, by Jearl 
Walker, published by John Wiley and Sons.  I checked there and this is the 
answer Walker has to your question.

“The constriction is sufficiently narrow for the mercury to pass it only 
under pressure, either from thermal expansion or from the “centrifugal” 
force when is is swung in an arc.  When cooling, the mercury thread breaks 
at the constriction because the intermolecular forces in the mercury are 
not strong enough to pull the upper column of mercury through the 

Walker includes several references.

I would add to this that a liquid is strong under compression but weak 
under tension.  In other words, while it is true that if you push on a 
liquid it will push back and resist being compressed, if you pull on a 
liquid it is possible to rip the liquid apart and form pockets of what is 
nearly a perfect vacuum.  This process is known as cavitation.  I suspect 
that your question about surface tension is relevant, because at some 
point, before the thread of mercury is completely broken, a small bubble 
most form somewhere (In the middle of the thread or at the wall?).  I am 
certain that the energetics involved in the formation of this original, 
small bubble must depend on surface tension.  I would guess that liquids 
with higher surface tension would require larger negative pressures in 
order to cavitate.  

A brief word about surface tension.  One way to think about this is to 
suppose you could form a thin liquid film between two identical circular 
metal wire rings that are close to one another and concentirc (centers 
lined up).  To make this experiment simple to think about, imagine 
performing it in the space shuttle, while in orbit.  Then there are no 
gravitational forces to consider.  You would find that the two rings are 
being pulled together with a force proportional to the circumference of the 
circular rings, and that different liquids would pull with differing 
forces.  This force is surface tension.  It has the units of force/length.    

Here is an experiment you might try concerning the fever thermometer.  Is 
it possible to cool the thermomether down very, very slowly so that the 
mercury thread does not break?  You may have to be very clever to think of 
a way to do this but I’m betting it can be done, thought it might take a 
very long time (tens of minutes, a few hours?).

I hope this has been of some help.  

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