In (88), ρ is the resistivity of the material, f, the frequency of operation, and i δ the skin depth. This formula comes from the observation that Diammin is the absolute minimum effective diameter of the helical coil, so Diam∞ (effective diameter at very high frequency) must lie somewhere between Diam0 and Diammin. Further, Θ is used as the weighting factor to evaluate the effective diameter on the specified frequency.