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A central part of the story of the discovery of superfluid3 He is the cooling technique used for the experiments, the Pomeranchuk Effect. Although it is not an especially useful technique for obtaining low temperatures today, it contains my favorite example of the use of the Clausius-Clapeyron equation. The cooling technique is fun to describe in undergraduate physics classes on thermodynamics.
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Robert C. Richardson (1997) studied this question.
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