hardening induction coil

Whenever someone is talking about induction heating, reference is often made to the phenomenon of skin effect.1 Skin effect is considered a fun- damental property of induction heating, representing a nonuniform distribution of an alternating current within the conductor cross section.

This effect will also be found in any electrically conductive body (workpiece) lo- cated inside an induction coil or in close proximity to the coil. According to this phenomenon, eddy currents induced within the workpiece will primarily flow in the surface layer (the "skin"), where 86% of all induced power will be concentrated.
This layer is called the reference depth or current penetration depth, 5. The degree of skin effect de- pends on the frequency and material properties (electrical resistivity, p, and relative magnetic permeability, of the conductor.1