The current that passes is measured when a unit EMF is eventually immersed in series in an infinitely long isolated conductor of zero resistance . The current equals the transient surge admittance as well as its reciprocal is the transient surge impedance . The surge impedance improves with time in a determinable manner from a small value . When such a conductor forms only the connecting leads to a pair of infinitely long parallel conductors , the surge impedance approaches the conventional value for the parallel lines .