XLVIII. Thermodynamic relations applicable near a lambda-transition

Abstract It is well known that there are not many examples of higher-order transitions which can be justifiably compared with Ehrenfest's theory of such transitions. The specific heat C p at a λ-transition does not usually exhibit a well-defined discontinuity (the superconducting transition is exceptional in this respect), but rather rises on the low-temperature side to a very high maximum whose ultimate height appears to be limited only by the shortcomings of experimental technique. This behaviour, while diminishing the practical value of Ehrenfest's method of approach, makes it possible to derive simple relations between various thermodynamic quantities which should be applicable in the vicmity of the λ-transition.