What effect does the use of magnetic shielding around the exciting coil generally have?

The use of magnetic shielding around the exciting coil is designed to contain the magnetic field generated by that coil. By doing so, the shielding effectively reduces the extent of the magnetic field's reach outside the coil. This containment decreases the magnetic field extension to areas that do not require magnetization or inspection, thus directing more of the magnetic field's energy into the targeted area.

As for eddy current penetration, the magnetic shielding acts to minimize the stray magnetic field, which influences the conductor’s response to the magnetic field passing through it. With a decreased magnetic field extension, this leads to reduced eddy current generation in the materials since the strength of the magnetic field directly influences the magnitude of the induced eddy currents. As a result, there is a decrease in the ability of eddy currents to penetrate the material under inspection.

In sum, magnetic shielding confines the magnetic field, leading to a decrease in both the magnetic field extension and the associated eddy current penetration effectively. This principle is significant in nondestructive testing, where controlled, localized magnetic fields are required for accurate assessment.