Skin depth (often denoted by lower-case Greek letter delta δ) - such a depth of penetration of alternating current or electromagnetic field, at which the amplitude is reduced to a value of 1/e (around 37%), as compared to the amplitude at the surface of the conductor.

Skin depth is a function of frequency of current, as well as magnetic permeability and electric resistivity (or conductivity) of the conductor in question.

For good conductors (like most metals) the function can be defined as:¹⁾

where: π - mathematical constant, f - frequency (Hz), μ_r - relative permeability of the conductor, μ₀ - magnetic permeability of free space (H/m), σ - electric conductivity (S/m), ρ = 1/σ - electric resistivity (Ω·m), ω = 2·π·f - pulsation frequency (rad/s).

• Skin effect
• Litz wire