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. It causes the effective resistance of the conductor to increase with the frequency of the current. The effect was first explained by Lord Kelvin in 1887. Nikola Tesla also investigated the skin effect. The skin effect has practical consequences in the design of electrical power transmission and distribution, and in radio-frequency and microwave circuits.
Mathematically speaking, the current density J in the conductor decreases exponentially with depth δ, as follows:
- J = e - δ / d
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:
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:
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 high-frequency transformers, to increase their efficiency.
Large power transformers will be wound with conductors of similar construction to Litz wire, but of larger cross-section.
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 silver-plated providing a better conductor (the best possible conductor excepting only superconductors) than copper on the 'skin' of the conductor. Silver-plating 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 |
See also