There are several reasons: even at low frequencies conduction pass outside the coil (i.e. Diama+Diamw) is longer than the inside pass (Diama- Diamw), also the stretching of wire, while producing the coil, increases the resistivity of the outside pass. So the effective loop diameter must be modified, due to the tendency of the current density to move towards the inside edge of the coil. This is particularly important due to Diam in (82) where the inductance is proportional to the square of Diam. In another study in Ref.[8], the effective current diameter for a low frequency case was evaluated as (87).