When does an eddy current signal get produced with a product of uniform cross-section?

An eddy current signal is produced when a conductive material is subjected to a changing magnetic field, which occurs when there are changes in the proximity of the test coil to the conductive material. In the case of a product with a uniform cross-section, an eddy current signal will be generated when either end of the product comes near the test coil.

The leading end approaching the test coil changes the magnetic field interaction, causing eddy currents to form in response to the alteration in the magnetic flux due to the approach. Similarly, when the trailing end comes near the test coil, the decreasing distance also results in a change of the magnetic field, generating a corresponding eddy current. This phenomenon happens because the coil induces a varying magnetic field, which leads to the creation of eddy currents in the conductive material.

Therefore, the correct conclusion is that both the leading end and the trailing end approaching the test coil will produce an eddy current signal. This comprehensive interaction illustrates how the dynamics of a uniform cross-section product relative to the test coil produces detectable signals due to the principles of electromagnetic induction involved in eddy current testing.