What do we call an eddy current test coil that uses magnetic material to shape the magnetic field?

The term used to describe an eddy current test coil that employs magnetic material to shape the magnetic field is known as a gap coil. This configuration utilizes the magnetic material to focus and direct the electromagnetic field, enhancing the sensitivity of the test and allowing for better interaction with the conductive material being tested.

Gap coils are specifically designed to improve the efficiency of the eddy current testing process by optimizing the field distribution. This feature helps in detecting surface and near-surface flaws more effectively, which is crucial for many non-destructive testing applications.

In contrast, other types of coils mentioned in the options serve different functions in eddy current testing. Bobbin coils, for example, are typically used for testing cylindrical parts and are not designed to focus the magnetic field with the help of magnetic materials. Encircling coils are designed to fully encompass the object under inspection but do not incorporate the same magnetic field shaping characteristics. Spinning coils are utilized in specialized applications and are not primarily focused on the shaping of magnetic fields like the gap coil.