If, on the other hand, the structure consists of two separate conductors as in the case of the parallel-wire and coaxial transmission lines shown in fig 5-27 the flux linkage A associated with a length l of either line refers to the flux O through a closed surface between the two conductors themselves but it may be ignored by assuming that currents flow only on the surfaces of the conductors vanishes this assumption is justified by the fact that our interest in calculating A is for the purpose of determining the inductance of a given stucture and inductance is of interest primarily in the ac case as we will see later in section 7-5 the current flowing in a conductor under ac