Beginning from the perspective of the one-port circuit elements described in the last chapter, it is not difficult to see how such elements can be generalized to a multidimensional setting. Consider again the inductance and its WD one-port equivalent, shown in Figure 3.4.
The voltage across the inductor is integrated and scaled by a factor
to yield a current. The fact that the signal flow graph for the WD one-port equivalent is causal indicates that the inductor one-port is associated with the forward time direction. In the multidimensional case, the concepts of a direction associated with a circuit element and causality become crucial.
