As indicated by the " " marks of Onsager (1931),<ref name="Onsager 1931 I"/> such a metaphorical but not categorically mechanical force, the thermal "force", <math>X_{th}</math>, 'drives' the conduction of heat. For this so-called "thermodynamic force", we can write | As indicated by the " " marks of Onsager (1931),<ref name="Onsager 1931 I"/> such a metaphorical but not categorically mechanical force, the thermal "force", <math>X_{th}</math>, 'drives' the conduction of heat. For this so-called "thermodynamic force", we can write |