The respective probe elements are available as a shielded and unshielded version. The shielding reduces the expansion of the magnetic field, so testing closer to material edges is possible. The five different probe elements can be used in a wide variety of probe shapes. The probe form can be adapted individually to the respective requirement. If, for example, an incorrect bolt has been installed in a machine, the probe shape can be adapted so that the bolt can be reached without dismounting the machine. For more frequent applications, test kits are available to guarantee a simple and, above all, safe procedure.