In my engineering for electricity class, the skinning effect was the resistance value of the material that electricity was supposed to flow across. That the difference between AC and DC when applied to metal was its frequency in ferrous and non-ferrous material due to the skinning effect, such as DC welding works better with non-ferrous materials like aluminum as a rod instead of iron.
I was taught that all electricity flows over the surface, whether it was AC or DC didn't matter. There could however be different resistance value depths depending on the material were penetration is involved; the difference was most likely nil, since that was never covered.
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