In a coil with both primary and secondary windings, what influences the voltage across the secondary?

The voltage across the secondary winding of a coil in Eddy Current Testing (ET) is influenced by several interrelated factors, which is why the correct answer encompasses all listed options.

Test material permeability plays a significant role because it affects how well eddy currents can be induced within the material being tested. A material with higher permeability allows eddy currents to flow more easily, which in turn increases the magnetic coupling between the primary and secondary windings, resulting in a higher voltage in the secondary winding.

Test frequency also has a direct impact. As frequency increases, the skin effect influences how deeply the eddy currents penetrate the material. Higher frequencies can lead to more significant changes in the magnetic fields between the windings, affecting the induced voltage. Essentially, varying the frequency alters the dynamics of how the magnetic field interacts with the test material, subsequently changing the secondary voltage.

Finally, the geometry of the test subject, including factors such as thickness, shape, and surface condition, can alter how eddy currents are generated and distributed. Changes in the geometry can affect the magnetic coupling and the efficiency with which the primary winding’s field induces currents in the material, thus impacting the voltage detected in the secondary winding.

Therefore, all these factors combined – permeability, frequency,