power of the cocked spring 109 through 111 then insures that the insert is ejected by the ejection element 103 through 105 from the inside of the insulating body at least to such degree that it can be easily grasped by hand at its front side and be withdrawn in a preferred embodiment each tappet also trips a switch particularly a microswitch in such manner that the current supply to the primary transmitter on the device side is interrupted when an insert is ejected as an exemplary embodiment fig8 shows the internal format of an insert 15 through 18 said internal format is characterized in that a total of two printed circuit boards 120 and 121 are mounted between the front part 118 of the insert and a back part 119 in the manner of a plug type connection the essential electronic components for energy and signal transmission are mounted on said printed circuit boards this partial structure comprised of front and back surface as well as printed circuit boards finally is covered in a sandwich manner by means of two metallic shielding plates 122 and 123 for example consisting of aluminum or nonmagnetic sheet steel both shielding plates 122 and 123 are provided with claws 124 or respectively 125 at the front edge when the respective plate is seated with flat edges 126 127 or respectively 128 129 on the inside shoulder of the front plate 118 the claws 124 or respectively 125 engage in recesses below the shoulder so that a type of clamped connection for supporting the front edge of the two plates 122 and 123 is produced at the shoulder the back edges 130 or respectively 131 of the plates.