Editing Skin effect (section)

{{Short description|Tendency of AC current flow in a conductor's outer layer}}
{{Redirect|Skin depth|the depth (layers) of biological/organic skin|skin}}
[[File:Skin depth.svg|thumb|Distribution of current flow in a cylindrical conductor, shown in cross section.  For [[alternating current]], current density decreases exponentially from the surface towards the inside. Skin depth, δ, is defined as the depth where the current density is just 1/e (about 37%) of the value at the surface; it depends on the frequency of the current and the electrical and magnetic properties of the conductor.]]
[[File:Induktionskochfeld Spule.jpg|thumb|200px| [[Induction cooker]]s use stranded coils ([[Litz wire]]) to reduce heating of the coil itself due to skin effect. The AC frequencies used in induction cookers are much higher than standard mains frequency ‒ typically around 25–50 kHz.]]

In [[electromagnetism]], '''skin effect''' is the tendency of an [[alternating current|alternating electric current]] (AC) to become distributed within a [[Conductor (material)|conductor]] such that the [[current density]] is largest near the surface of the conductor and decreases exponentially with greater depths in the conductor. It is caused by opposing [[eddy current]]s induced by the changing [[magnetic]] field resulting from the alternating current. The electric current flows mainly at the ''skin'' of the conductor, between the outer surface and a level called the '''skin depth'''.

Skin depth depends on the [[frequency]] of the alternating current; as frequency increases, current flow becomes more concentrated near the surface, resulting in less skin depth. Skin effect reduces the effective cross-section of the conductor and thus increases its effective [[electrical resistance|resistance]]. At 60 [[Hertz|Hz]] in copper, skin depth is about 8.5&nbsp;mm. At high frequencies, skin depth becomes much smaller.

Increased AC resistance caused by skin effect can be mitigated by using a specialized multistrand wire called [[litz wire]]. Because the interior of a large conductor carries little of the current, tubular conductors can be used to save weight and cost.

Skin effect has practical consequences in the analysis and design of radio-frequency and [[microwave]] circuits, [[transmission line]]s (or waveguides), and [[Antenna (radio)|antenna]]s. It is also important at [[Mains electricity|mains frequencies]] (50–60&nbsp;Hz) in AC [[electric power transmission]] and distribution systems.  It is one of the reasons for preferring [[high-voltage direct current]] for long-distance power transmission.

The effect was first described in a paper by [[Horace Lamb]] in 1883 for the case of spherical conductors,<ref>{{Cite journal | last=Lamb|first=Horace|date=1883-01-01|title=XIII. On electrical motions in a spherical conductor | journal=Philosophical Transactions of the Royal Society of London | volume=174| pages=519–549| doi=10.1098/rstl.1883.0013| s2cid=111283238}}</ref> and was generalized to conductors of any shape by [[Oliver Heaviside]] in 1885.