As skin effect increases, the currents are concentrated near the outside the inner conductor (r = a) and the inside of the shield (r = b). Since there is essentially no current deeper in the inner conductor, there is no magnetic field beneath the surface of the inner conductor. Since the current in the inner conductor is balanced by the opposite current flowing on the inside of the outer conductor, there is no remaining magnetic field in the outer conductor itself where b < r < c . Only L ext contributes to the electrical inductance at these higher frequencies.