What is the term used for a surface coil that can be rotated about the circumference of a test specimen?

The term that describes a surface coil capable of rotating around the circumference of a test specimen is "spinning coil." This type of coil is specifically designed to enhance the electromagnetic coupling with the test specimen from various angles, thereby providing more comprehensive and accurate measurements. The ability to rotate allows for an improved assessment of the specimen’s condition over its entire surface area, helping in the detection of defects that may not be observable from a static position.

Other types of coils, such as a bobbin coil, encircling coil, or gap coil, serve different functions. A bobbin coil typically refers to a coil that is wound around a core or bobbin and is often used for applications where the coil needs to be inserted into the specimen, rather than revolving around it. An encircling coil is designed to surround the test object but does not allow the kind of rotational movement the spinning coil does. A gap coil usually refers to setups where there is a deliberate gap in the coil design to measure specific conditions rather than full surface assessment.