the use of a sliding contact 46 reduces the sensitivity of the measurement process according to the invention to the contact resistance relationships in the area of the seals between the piston 12 and the cylinder housing 14 otherwise the method of operation of the second embodiment is comparable to that of the first embodiment according to fig1 there are two evaluation arrangements at the positions 38 and 46 fig3 shows a third embodiment of the invention that is comparable to the second embodiment shown in fig2 accordingly the evaluation arrangement 40 is provided with several input channels moreover the detection arrangement 36 is provided with two further devices for the evaluation of a measurement voltage at the cylinder 16 in addition to the electrical line 42 which connects a third position 38 with the evaluation arrangement 40 this is an electrical line 49 on the one hand that connects a fourth position 50 with the evaluation arrangement 40 on the other hand a fifth position 52 is connected to the evaluation arrangement 40 by an electrical line 54 correspondingly the voltages prevailing at each point are evaluated as measurement voltages and directed to the evaluation unit 40 over the input channels 1 to 3 simultaneously at three positions along the cylinder housing 14 particularly at the third.