Diathermic border

The diathermic border has no volume. So it cannot take up volume energy. The diathermic border faces a temperature Th from one surface and a temperature Ti from the other surface. Here Th > 7]. When entropy d5(7/,) passes into the border across a surface, the diathermic border ejects -d5(7]) at the opposite surface following the relation... 

In this way, the diathermic border is a generator for entropy, even when it cannot store entropy. Moreover, the diathermic border acts as a valve for entropy. If 7), < 7] then it does not allow an inflow of entropy, d CT),). 

Also, the porous plug in the Joule - Thomson experiment is an idealized device. It has certain properties, such as lack of volume, common to the diathermic border, but it does not allow a flow of entropy without flow of matter. When matter flows though the porous plug, the pressure and the temperature are reduced. This is associated with an increase of entropy. 

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