I'm sure you are all familiar with calculating the inductance of a long transmission line. We first calculate the partial self inductance and we add to the partial mutual inductance due to the current in the other conductors. Looking at the image of a single-phase system, where I1 + I2 = 0. The partial inductance per unit of length of wire 1 is given by this equation: Here is my question: Why do we estimate the MUTUAL flux linkage as the flux going through the GREEN area in the image and not the RED area? I don't see how the flux outside both wires accounts for the mutual inductance of these two wires.