In order to generate measurable eddy currents, the test specimen must be:

To generate measurable eddy currents, the test specimen must be a conductor. This is because eddy currents are induced in conductive materials when they are exposed to a changing magnetic field. The phenomenon occurs due to the electromagnetic induction principle, which states that a changing magnetic field can induce circulating currents within a conductor.

In the context of eddy current testing, materials such as metals allow for the flow of electric currents. When an alternating magnetic field is applied to a conductive material, the resulting eddy currents create their own magnetic fields, which can be measured to evaluate the material's properties or detect discontinuities. Since insulators do not allow electric current to flow, they cannot support the formation of eddy currents. Ferromagnetic materials, while conductive, may have properties that influence the behavior of eddy currents, but the fundamental requirement remains that the material must be a conductor to generate measurable eddy currents.