Which factor does not affect the inductance of an eddy current test coil?

In the context of eddy current testing, inductance is primarily influenced by factors related to the coil's physical characteristics and its interaction with the tested material. The choice of test frequency indeed plays a crucial role in the eddy current testing process, affecting the penetration depth and the eddy currents induced in the material. However, it does not impact the intrinsic inductance of the coil itself.

The other factors, such as the diameter of the coils, the overall shape of the coils, and the distance from other coils, directly affect the magnetic field generated by the coil, which in turn influences the inductance. The diameter of the coils can change the inductive properties because a larger diameter tends to increase the inductive reactance, while the shape of the coils can influence how the magnetic field interacts with the test material, further altering the inductance. Additionally, the distance from other coils can also affect inductance due to mutual inductance effects, where the magnetic field from one coil interacts with another.

Since the test frequency does not inherently change the inductance of the coil but rather affects the testing parameters and how the eddy currents behave in the test material, this makes it the correct choice for the factor that does not affect inductance.