Process for contactless measurement of the electrical resistance of a test material

Control device for controlling a windshield wiper of a motor vehicle, comprising a sensor consisting of at least two coupling electrodes, at least two contact elements coupled to the coupling electrodes, means for applying an alternating voltage to a windshield by the two coupling electrodes and thus, capacitively coupling an alternating current into the windshield at at least three different frequencies via the two coupling electrodes, an evaluation circuit connected to contact elements as means for detecting a complex total impedance, that corresponds to a coupling capacitance between the coupling electrodes, a stray capacitance between the coupling electrodes, and the resistance of the windshield and any water thereon, acting between the coupling electrodes at said at least three different frequencies as a function of a water-influenced, capacitively coupled alternating current and a capacitance, which is affected by the dielectric properties of water, measured between the coupling electrodes;

• wherein said means for detecting comprises means for measuring said total complex impedance and for determining the stray capacitance between the two coupling electrodes, the coupling capacitance between the two coupling electrodes and the windshield as well as the resistance of the windshield and any quantities of water thereon from the total complex impedance measured, and means for controlling operation of the windshield wiper as a function of the amount of any quantities of water on the windshield and the manner in which said water is distributed thereon.