a pair of alternate marker elements the first marker element of one polarity will set ff 125 a subsequent signal applied to ff 125 of the same polarity will have no effect the next signal of oppositve polarity will reset ff 125 if threshold stage 31 therefore provides two subsequent signals of the same polarity ff 146 will respond to provide a recognition output at terminal 149 and hence on line 150 integrater 121 is reset upon simultaneous occurrence of a signal at output 126 of ff 125 and a signal at the output of threshold stage 32 this circuit avoids problems of drift of the integrator if the input voltages deviate from null the present invention provides a circuit which is simple thus inexpensive and reliable the recognition is based on recognizing differences in characteristics of the transducing elements that is differences in direction of magnetization of the permanent magnet elements 21 22 or 66 67 68 69 respectively the wave shaping and recognition stages connected to the transducer system formed by the combination of the marker elements and the transducer 23 are used to recognize the respective marker elements and to process the pulses triggered by the respective marker elements various instants.