Skin depth


 * dashed black (Mn-Zn) - magnetically soft ferrite 3C90
 * light grey (Al) - metallic aluminium
 * orange (Cu) - metallic copper
 * green (steel 410) - magnetic stainless steel, grade 410
 * blue (Fe-Si) - grain-oriented electrical steel
 * dark grey (Fe-Ni) - high-permeability permalloy (80%Ni-20%Fe)

Skin depth - such a depth of penetration of alternating current, at which the amplitude is reduced to a value of 1/e (around 37%), as compared to the current 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:


 * $$\delta = \frac{1}{\sqrt{\pi \cdot f \cdot \mu \cdot \sigma}} = \sqrt{\frac{2 \cdot \rho}{\omega \cdot \mu}}$$

where: &pi; - mathematical constant, f - frequency (Hz), &mu; = &mu;0 &middot; &mu;r - magnetic permeability (H/m), &sigma; - electric conductivity (S/m), &rho; = 1/&sigma; - electric resistivity (&Omega;&middot;m), &omega; = 2&middot;&pi;&middot;f - pulsation frequency (rad/s)