amplified in the amplifier 13 which comprises a higher amplification factor than the amplifier 14 the signal passing from the calibration logic to the store via the switch 43 activates a second storage region ii to which corresponds according to fig4 a calibration curve ii which is developed more steeply in contrast to the curve of the difference frequency and is shown in fig4 in broken lines as described above during the movement of the measured value emitter 1 back from the standard object 39 storage is now again effected in the store 21 only in an operative region lying between the abscissa and the line g in fig4 the storage regions overlap somewhat at their boundaries i e the measured values and positions of the measured value emitter lying in these regions are stored in both parts i and ii of the store and can be read therefrom selectively in order to carry out measurement the measured value emitter 1 according to fig1 is brought up to the cable 4 5 to be examined the distance of the measured value emitter from the conductive 5 of the cable corresponds to the thickness of the cable insulation 4 the frequency fm of the measuring oscillator also corresponds to this distance the above mentioned zero adjustment is carried out with the uninfluenced measured value emitter as necessary before the measurement for the measurement the switch 43 is then shifted into the illustrated position and the switch 29 is opened the relay 42 and the region of the store are accordingly now controlled from the frequency discriminator 9 and the comparator 20 is ineffective.