an oil soaked cloth over roller 10 a measuring device 20 has been arranged next to roller 10 the measuring device 20 contains a capacitive sensor that can determine the known distances on the basis of capacitive changes in the environment of its measuring transducer inductive or electro optical sensors can likewise be used measuring device 20 is connected with a combined computer control unit 40 in order to measure the thickness c of liquid layer 12 in this case the oil film the electrical capacitance between a measuring transducer of the sensor and the roller surface without a liquid layer 12 represented by the line segment a is measured with a capacitive sensor of the measuring device 20 the electrical capacitance between the roller surface and the measuring transducer is now clearly determined and is stored in the computer 41 the next step as described above is to apply the liquid layer 12 of the oil film to the roller 10 the relative permittivity u03b5 of liquid layer 12 is different from that of the roller surface so that the measurement of the capacitive sensor now detects another capacitance between the measuring transducer of the sensor of measuring device 20 and the surface of liquid layer 12 represented by.