Pertains to a film element of a soap film changing in area at constant mass and is the differential change of its surface tension with relative change in area \(A\), \[E=A\ (\frac{\partial \sigma }{\partial A})_{T,p,n_{i}}\] where \(T\) is the thermodynamic temperature, \(p\) is the pressure, and \(n_{i}\) is the amount of substance of the species \(i\).