If the test frequency increases while the field strength remains constant, what happens to the surface eddy current density?

When the test frequency increases while the field strength remains constant, the surface eddy current density actually increases. This is primarily due to the skin effect, which causes eddy currents to flow more densely near the surface of a conductive material as frequency rises.

At higher frequencies, the alternating current penetrates less deeply into the conductor, leading to a concentration of currents at the surface. As a result, the density of these eddy currents—essentially the amount of electric current flowing per unit area at the surface—increases. Therefore, when the frequency is increased while maintaining the same field strength, more eddy currents are generated closer to the material's surface, resulting in an increase in surface eddy current density.

This principle is essential for understanding non-destructive testing techniques, particularly in electromagnetic methods where detecting surface flaws or characteristics is critical.