Skin effect
From Academic Kids

The skin effect is the tendency of an alternating electric current to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor.
Contents 
Introduction
The skin effect causes the effective resistance of the conductor to increase with the frequency of the current. The effect was first described in a paper by Horace Lamb in 1883 for the case of spherical conductors, and was generalized to conductors of any shape by Oliver Heaviside in 1885. The skin effect has practical consequences in the design of electrical power transmission and distribution, and in radiofrequency and microwave circuits.
Mathematics
Mathematically speaking, the current density J in an infinitely thick plane conductor decreases exponentially with depth δ from the surface, as follows:
 <math>J=e^{{\delta /d}}<math>
where d is a constant called the skin depth. This is defined as the depth below the surface of the conductor at which the current is 1/e (about 0.37) times the current at the surface. It can be calculated as follows:
 <math>d=\sqrt{{2\rho}\over{\omega \mu}}<math>
where
 ρ = resistivity of conductor
 ω = angular frequency of current = 2π × frequency
 μ = absolute magnetic permeability of conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current. For long, thin conductors such as wires, the resistance is approximately that of a hollow tube with wall thickness d carrying direct current. For example, for a round wire, the resistance is approximately:
 <math>R={{\rho \over d}\left({L\over{\pi (Dd)}}\right)}\approx{{\rho \over d}\left({L\over{\pi D}}\right)}<math>
where
 L = length of conductor
 D = diameter of conductor
The final approximation above is accurate if D >> d.
Mitigation
A type of cable called litz wire (from the German Litzendraht, woven wire) is used to mitigate the skin effect for frequencies of a few kilohertz to about one megahertz. It consists of a number of insulated wire strands woven together in a carefully designed pattern, so that the overall magnetic field acts equally on all the wires and causes the total current to be distributed equally among them. Litz wire is often used in the windings of highfrequency transformers, to increase their efficiency.
Large power transformers will be wound with conductors of similar construction to Litz wire, but of larger crosssection.
In other applications, solid conductors are replaced by tubes, which have the same resistance at high frequencies but of course are lighter.
Solid or tubular conductors may also be silverplated providing a better conductor (the best possible conductor excepting only superconductors) than copper on the 'skin' of the conductor. Silverplating is most effective at VHF and microwave frequencies, because the very thin skin depth (conduction layer) at those frequencies means that the silver plating can economically be applied at thicknesses greater than the skin depth.
Examples
In a copper wire, the skin depth at various frequencies is shown below.
frequency  δ 
60 Hz  8.57 mm 
10 kHz  0.66 mm 
10 MHz  21 µm 
In Engineering Electromagnetics, Hayt points out that in a power station a bus bar for alternating current at 60 Hz much more than 1/3rd of an inch (8 mm) thick is wasteful of copper, and in practice bus bars for heavy AC current are rarely more than 1/2 inch (12 mm) thick except for mechanical reasons. A thin film of silver deposited on glass is an excellent conductor at microwave frequencies.
See also
 surface wave
 Skin Effect and HiFi Cables (http://www.standrews.ac.uk/~www_pa/Scots_Guide/audio/skineffect/page1.html)
References
William Hart Hayt, Engineering Electromagnetics Third Edition,(1974), McGraw Hill, New York ISBN 070273901
Paul J. Nahin, Oliver Heaviside: Sage in Solitude, (1988), IEEE Press, New York, ISBM 0879422386de:Skineffekt ja:表皮効果 nl:Skineffect it:Effetto pelle vi:Hiệu ứng bề mặt