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  When an AC current flows in a coil in close proximity to a conducting surface the magnetic field of the coil will induce circulating (eddy) currents in that surface. The magnitude and phase of the eddy currents will affect the loading on the coil and thus its impedance.  
  Electrical conductivities and magnetic permeabilities may be related to structural features such as hardness, chemical composition, grain size and material strength. An important advantage of eddy current testing over some other methods, such as ultrasonic, magnetic particle and potential drop techniques, is that there is no need for physical contact with the surface of the object being tested. Thus, careful surface preparation is unnecessary (other than the removal of metallic adherents).
The practice of eddy current testing, consists of exciting an alternating current at given frequencies through a coil, often called a probe coil or simply a probe, located as near as possible to the electrically conducting object being tested, and thus to induce eddy currents in the object . As a result changes take place in the components of the coil’s impedance, which can be related to the design of the coil, the size, shape and position of the test object and the values of its magnetic permeability ? and electrical conductivity ? of the object. Also some harmonics will be generated due to nonlinearity of the parameters. The value of electrical conductivity of a metal depends on several factors, including its chemical composition, the nature of its crystalline structure, its mechanical properties temperature, as well as its electrical properties. When using eddy currents to measure conductivity it is important, for the sake of correctness and accuracy, to ensure that these factors are kept under control, along with other factors such as geometry, the magnetic permeability, the temperature of the specimen and the temperature and lift-off of the probe.
The simplest type of probe is the single-coil probe. However, sometimes it is desirable to use a probe consisting of two coils arranged in transformer fashion and therefore known as a transformer probe. Here the primary coil induces eddy currents in the test object and the secondary coil acts as a detector. The use of this probe provides an enhanced signal-to-noise ratio for detection. The radii of the tubes and rods under test determine the radii of encircling and internal axial coils.
   The speed and precision of scanning can be improved with the use of array of probes that are suitably spaced to allow for complete surface coverage.
Before any eddy current test, a calibration should be made with standard samples. Knowledge of those relationships, which are relevant to a given test, is necessary for calibration.
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