If an alternating current flows through an electrical conductor, the skin effect reduces the current density
inside the conductor, while it is highest at the conductor surface. The decrease in current density towards the interior of the conductor is determined by the penetration depth (small delta), skin depth or penetration depth. It is an exponential quantity determined from the current density at the conductor surface. Penetration depth is defined by 1/e, with "e" equal to 2.71, as the 37% value of the current density at the conductor surface. The frequency (f) is inversely proportional to the calculation of the penetration depth, as are the conductivity and permeability of the conductor material.