Which type of coil primarily relates to the fill factor term?

The correct choice regarding the type of coil that primarily relates to the fill factor term is an encircling coil. This type of coil is designed to completely wrap around or encircle the part being inspected. The fill factor refers to the effectiveness of the coil in covering the area of interest within the part, and it significantly impacts the sensitivity and resolution of the electromagnetic testing. An encircling coil maximizes the area of contact with the test specimen, thereby enhancing the detection of flaws and improving the overall performance of the inspection.

In contrast, surface coils may not fully encircle the area of interest, potentially leading to a lower fill factor and reduced sensitivity. Coaxial cables, while essential in the transmission of signals, do not directly relate to the fill factor term in the context of coils used for testing. Flat coils also have limitations regarding the area they cover compared to encircling coils, which are designed specifically to optimize the measurement and testing process.