We introduce a non-linear theory of thermodynamic circuits in non-equilibrium
stationary states. The non-equilibrium conductance matrix of a composite device
is obtained from the ones of its sub-devices. This generalizes to thermodynamic
devices the concept of equivalent impedance defined in electronics. An abstract
example and the serial connection of two thermoelectric generators (TEG) with
constant thermoelectric coefficients are considered. Interestingly, a
current-dependent electrical resistance emerges from this connection.Comment: 5 pages, 3 figure