While shown in figure there are basically two single phase line conductor is presented.One conductor is the return circuit for the additional, this implies that in case the current in conductor one is I and then the current in conductor 2 is -I. 1st we consider conductor 1. The current flowing in the conductor can set up flux lines. However, the flux beyond a distance D r2 from the center of the conductor links a net current of zero as well as should not contribute to the flux linkage of the circuit. Too at a distance lower than D - r2 from the center of conductor 1 the current flowing through this conductor links the flux. Moreover since D >> r2 we can easily make the following approximations.