Abstract: A gradiometer for determining electrical conductivity of a medium contained in a containment, comprising: at least a first electrical coil, a second electrical coil and a third electrical coil, wherein the first coil is embodied as a transmitting coil, wherein the second and third coil are embodied as receiving coils; and a holding apparatus. The first, second and third coils are wound on the holding apparatus. The holding apparatus is arranged through a containment opening in a containment wall in such a manner that the first coil is positioned centrally with respect to the containment wall. The second and third coils are arranged mirror symmetrically to the first coil and are inductively coupled with the first coil.

Abstract: A two-part sensor composed of a sensor head and a sensor-head counterpart, testing is performed as to whether a correct sensor head is connected with the sensor-head counterpart.