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Designing a Thermometer to Measure Negative Temperature

My homework problem says to construct a thermometer which could accurately measure a negative temperature. I have no idea how to do this. Any help would be very appreciated!
 
ummm... mercury thermometers measure negative temperatures. So do thermocouples and Pt100/1000s.

what are you going for? How negative and in what unit °C/°F/K(can't be negative, but hey)?
 

jambaugh

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Hmmm.... interesting question. My first thought is that negative temperature systems (such as excited lasing media) are not stable and won't come into equilibrium with a thermometer so you couldn't measure their temperature in this way.
 

jambaugh

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ummm... mercury thermometers measure negative temperatures. So do thermocouples and Pt100/1000s.

what are you going for? How negative and in what unit °C/°F/K(can't be negative, but hey)?
My assumption is that he is referring to negative absolute temperature. This describes a system where the entropy decreases as the total energy increases. The arch-example is where you reverse the magnetic field for a system of spins which have previously been cooled near absolute zero. But the most common example occurs in pumped lasers. It is in this negative temperature regime where you get actual amplification.

It is not temperature in the usual sense of equilibrium with the environment but in the statistical mechanics sense and in the direction of flow of heat sense. Negative temperature systems tend to cool further until the temp jumps past -infinity.
 

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