Which of the following is not a factor that affects the inductance of an eddy current test coil?

In the context of factors that affect the inductance of an eddy current test coil, the test frequency is indeed an important parameter in the operation of eddy current testing, but it does not directly impact the inductance of the coil itself. Inductance is primarily determined by physical characteristics of the coil, such as the diameter of the coils, the overall shape, and the distance from other coils.

The diameter of the coils influences the magnetic field generated and can affect the coil's inductance. Similarly, the overall shape can determine how the magnetic field interacts with the test material. Lastly, the distance from other coils affects the coupling of electromagnetic fields, which can naturally have an influence on inductance as well.

On the other hand, while changing the frequency impacts the eddy currents themselves and the overall testing approach, it does not change the inherent inductance of the coil. Therefore, it is correctly observed that the test frequency is not a factor that affects the inductance of an eddy current test coil.