The so-called eddy current is the application of the basic theory of "electromagnetism" as the basis for conductor detection. Eddy current originates from electromagnetic induction. When alternating current is applied to a conductor, a magnetic field will generate a magnetic field inside and around the conductor. Eddy current is an induced current flowing in a loop. It is called "vortex" because its flow mode is the same as that of liquid or gas around obstacles in the loop. If a conductor is placed in a changing magnetic field, an eddy current will be generated in that conductor, and the eddy current will also generate its own magnetic field, which will expand with the increase of AC current. Therefore, when measuring some properties of metal materials, it will affect the intensity and distribution of eddy current, so that we can detect the changes of eddy current together, and then indirectly know whether the properties of metal (material, conductivity, hardness) have changed. That is, the change of magnetic flux.