104 which gauges are firmly attached to the hinge strips 98 99 by adhesive as can be noted from fig9 the pressure plate 102 has a recess 105 which is arranged on one side of the line 96 and through which a rod 106 extends with clearance the rod 106 is firmly attached to a cylindrical core 107 which is surrounded with clearance by a wire winding 108 the latter terminates in two electrical connections not shown the rod 106 is received at one end with clearance in a bore hole 109 provided in the frame 95 the other end of the rod 106 is mounted with clearance in a bearing 110 which is firmly attached to the plate 93 a pressure plate 111 is also mounted on the rod 106 preferably by pressfitting and thus follows the movements of the rod 106 as can be noted from fig1 a plate 112 is arranged parallel to the plate 93 the two plates 93 and 112 being connected to each other via cylindrical spacers 113 by means of screws 114 as can be noted from the lower part of fig9 the plate 112 extends merely far enough so that the electromagnet 94 is positioned enclosed between the two plates 93 and 112 in accordance with fig1 the pressure plates 102 and 111 are circular each of them having a guide edge 115 which prevents any twisting of the pressure plates 102 and 111 the angularly bent parts 100 and 101 formed on the pressure plate 102 are dimensioned so that the pressure plate 102 can be shifted between the two plates.