Eddy Current Testing Level I Practice Test 2026 – Complete Exam Prep

In Eddy Current Testing, skin depth is how far the eddy currents can penetrate into a conductor before they decay significantly. As the excitation frequency goes up, the currents are pushed toward the surface and penetrate less deeply. This is why higher frequency reduces skin depth.

The standard formula is δ = sqrt(2/(ω μ σ)), where ω = 2πf, μ is the magnetic permeability, and σ is the electrical conductivity. So when frequency f increases, δ decreases roughly in proportion to 1/√f. The material properties also matter: higher conductivity or higher permeability shrink the skin depth.

This is why the option stating that higher frequency reduces skin depth and giving δ = sqrt(2/(ω μ σ)) is the correct description. The other forms either misstate how frequency affects penetration or give incorrect formulas.