Alternating current flows through the coil inducing magnetic fields. If these fields are close enough to a work-piece, the magnetic fields will penetrate the work-piece. If the work-piece is a conductor the magnetic fields will induce currents (eddy-currents) in the work-piece. The coil is said to be the "driving coil" as it drives the fields into the work-piece, i.e. induces the fields in the work-piece.
The induced eddy-currents in the work-piece induce their own magnetic fields. These fields tend to oppose the fields generated by the driving coil, which changes the resistance in the probe. This resistance change is in the form as absolute resistance, plus a phase angle, known as impedance, so the coil not only acts as the driving coil, it also acts as the "sensing coil".
Most eddy-current probes have a driving coil that also acts as the sensing coil. However, some probes have one or more special sensing coils which sense impedance or voltage differences. These are usually referred to as differential probes.
The magnetic fields generated by the driving coil do not penetrate deeply into
the work-piece as the conductor acts as a shield. This limits the detection
(sensing) capabilities of most eddy-current probes to sensing near surface anomalies.
To help drive the fields deeper in the work-piece, ferrite (a non-conduction
ferrous material) is added to the probes. The ferrite alters the shape of the
magnetic fields. It tends to "focus" the fields giving better penetration
and finer discrimination of anomalies. Ferrite can be added in the core of the
coil, as a shield, or even surrounding the entire coil.
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