Basically, one distinguishes between two main groups of probes: coaxial probes and surface probes. In the case of coaxial probes one distinguishes between encircling coils, which surround the product to be tested and through which the product is fed (e.g. inspections of rods with surrounding test coils), and internal coaxial coils, which are enclosed by the product to be tested; i.e. the coil is fed through the product to be tested (e.g. inspections of pipe interiors). Coaxial probes always cover a full circumferential section of the product to be tested, either external or internal.