Eddy currents induced in a material by a changing magnetic field
from a coil concentrate near the surface of the material adjacent to the coil.
The strength of the eddy currents decays with depth because eddy currents
produce their own magnetic field that opposes the primary field. Therefore, the
eddy currents that form first at the surface, weaken the magnetic field that
induces the next deeper layer of eddy currents. This weakening of the magnetic
field and the resulting eddy currents continues with increasing depth into the
material. The decay of the eddy current density is exponential and is described
by the equation to the right.

Jx 
= 
Current Density
(amps/m^{2}) 
J_{o} 
= 
Current Density at Surface
(amps/m^{2)} 
e 
= 
Base Natural Log = 2.71828 
x 
= 
Distance Below Surface 
d 
= 
Standard Depth of Penetration 

This equation shows that the eddy current density (J) at some
depth (x) is a function of the density at the surface (J_{o}) and decays
exponentially with increasing distance from the surface and decreasing standard
depth of penetration. The standard depth of penetration term takes into account
the magnetic permeability and electrical resistivity of the material, and the
drive frequency of the test coil. An increase in these three values will result
in a decrease in the standard depth of penetration and this will result in
weaker eddy currents at the given depth.
In practice the actual eddy current density is rarely calculated
but it is important to know how the density or strength relates to the standard
depth of penetration. From the table below it can be seen that at a distance
below the surface of one standard depth of penetration, the strength of the eddy
currents will be 37% the strength of those at the surface. At a distance equal
to three standard depths of penetration the strength of the eddy currents are
only 5% the strength as those at the surface. As the distance below the surface
increase, the strength of the eddy currents continue to decay exponentially.
