Eddy-current testing
In 1879, another English-born scientist, David Edward Hughes, demonstrated how the properties of a coil change when placed in contact with metals of different conductivity and permeability, which was applied to metallurgical sorting tests.
[1] Much of the development of ECT as a nondestructive testing technique for industrial applications was carried out during World War II in Germany.
Professor Friedrich Förster while working for the Kaiser-Wilhelm Institute (later the Kaiser Wilhelm Society) adapted eddy current technology to industrial use, developing instruments measuring conductivity and sorting mixed ferrous components.
After the war, in 1948, Förster founded a company, now called the Foerster Group where he made great strides in developing practical ECT instruments and marketing them.
[2] Eddy current testing is now a widely used and well understood inspection technique for flaw detection, as well as thickness and conductivity measurements.
A different, albeit physically closely related challenge is the detection of deeply lying flaws and inhomogeneities in electrically conducting solid materials.
If the material contains a crack or flaw which make the spatial distribution of the electrical conductivity nonuniform, the path of the eddy currents is perturbed and the impedance of the coil which generates the AC magnetic field is modified.
Since the eddy currents are generated by an AC magnetic field, their penetration into the subsurface region of the material is limited by the skin effect.
The applicability of the traditional version of eddy current testing is therefore limited to the analysis of the immediate vicinity of the surface of a material, usually of the order of one millimeter.
Attempts to overcome this fundamental limitation using low frequency coils and superconducting magnetic field sensors have not led to widespread applications.
If the magnet is passing by a defect, the Lorentz force acting on it shows a distortion whose detection is the key for the LET working principle.
