Which type of coil produces a field that encircles the test specimen?

The type of coil that produces a field encircling the test specimen is the encircling coil. This design allows for the generation of a magnetic field that completely surrounds and interacts with the material being tested.

Using an encircling coil is particularly effective for detecting flaws or inconsistencies in the material, as the magnetic field can penetrate the entire surface and assess the integrity from all angles. This leads to a more thorough inspection, enabling the detection of surface and near-surface defects that might not be as easily identified with other coil configurations.

Other types of coils, such as the bobbin coil, primarily focus on generating a field within the test specimen or are used for specific applications that do not require a full surrounding field. Spinning coils and gap coils also have unique functionalities but do not encircle the specimen in the same comprehensive manner as the encircling coil. Therefore, the encircling coil's design and functionality make it the appropriate choice when the goal is to envelop the test piece completely with the magnetic field.