This is only the case in AC (skin effect). DC penetrates the entire conductor. The higher the frequency the less you penetrate the material (self-induction in

This is only the case in AC (skin effect). DC penetrates the entire conductor. The higher the frequency the less you penetrate the material (self-induction in the material forces the electrons outwards). This enables you to effectively do a sweep of material composition by increasing frequency and measuring the resistance. In theory you could even tell if the core was not conductive at all (due to the resistance not changing despite lower frequency). So even if the plating was thick, you would be able to tell because the resistance curve begins to change at lower frequencies due to Tungsten having a higher resistance. If your device is really accurate you could even measure the rough area of the core since skin effect is proportional to the square of the resistivity.