Editing Skin effect (section)

== Material effect on skin depth ==

In a good conductor, skin depth is proportional to square root of the resistivity.  This means that better conductors have a reduced skin depth.  The overall resistance of the better conductor remains lower even with the reduced skin depth. However the better conductor will show a higher ratio between its AC and DC resistance, when compared with a conductor of higher resistivity. For example, at 60&nbsp;Hz, a [[Circular mil|2000 MCM]] (1000 square millimeter) copper conductor has 23% more resistance than it does at DC.  The same size conductor in aluminum has only 10% more resistance with 60&nbsp;Hz AC than it does with DC.<ref>{{citation |editor-first=Donald G. |editor-last=Fink |editor-first2=H. Wayne |editor-last2=Beatty |title=Standard Handbook for Electrical Engineers |edition=11th |publisher=McGraw Hill |year=1978 |page=Table 18–21 }}</ref>

Skin depth also varies as the inverse square root of the [[Permeability (electromagnetism)|permeability]] of the conductor.  In the case of iron, its conductivity is about 1/7 that of copper.  However being [[ferromagnetic]] its permeability is about 10,000 times greater.  This reduces the skin depth for iron to about 1/38 that of copper, about 220 [[micrometre|micrometer]]s at 60&nbsp;Hz.  Iron wire is impractical for AC power lines (except to add mechanical strength by serving as a core to a non-ferromagnetic conductor like aluminum). Skin effect also reduces the effective thickness of [[lamination]]s in power transformers, increasing their losses.

Iron rods work well for [[Direct current|direct-current]] (DC) [[welding]] but it is difficult to use them at frequencies much higher than 60&nbsp;Hz.  At a few kilohertz, an iron welding rod would glow red hot as current flows through the greatly increased AC resistance resulting from skin effect, with relatively little power remaining for the  [[Arc welding|arc]] itself.  Only [[non-magnetic]] rods are used for high-frequency welding.

At 1 megahertz skin effect depth in wet soil is about 5.0&nbsp;m; in seawater it is about 0.25&nbsp;m.<ref>{{harvnb|Popovic|Popovic|1999|p=385}}</ref>