To illustrate the distribution of eddy current density in direction of depth a layer model can be used.
Directly at the surface of the test object eddy currents are induced, whose direction is mirror-inverted to the coil current and which form a magnetic field in opposition to the coil field. Corresponding to Ohm’s Law, real losses of energy can be noticed in the test object (strictly speaking: a small portion of energy is transformed to heat).
So, the opposing magnetic field is weaker than the excitation field of the coil. From the superimposition of them both a weakened overall magnetic field results; in practical terms the magnetic field of the coil is strongly shielded in the depth direction.
In the imaginary layer underneath, this weakened magnetic field now induces less eddy currents, whose direction again reverses. On their part, these eddy currents act like a coil and induce in turn eddy currents in the next lower layer.
This process continues on in the direction of depth. In this way, and with increasing distance from the surface, the excitation field penetrating the test object becomes increasingly weakened and out of phase due to the flow of the eddy currents. The phase shift corresponds to the time lag that results from the limited speed of the eddy current flow.
That is why the displacement of the magnetic field and the concentration of the eddy currents at the surface of the test specimen is called the skin effect.
A measure for the decline in density of the eddy current with increasing depth is the standard depth of penetration. The rising time lag of the flowing eddy currents that comes with increasing distance from the surface leads to a depth-dependent phase shift of the eddy currents.