1. A method of determining an interference magnetic field component in a motor vehicle provided with an electronic compass, the compass having a fixedly mounted magnometer consisting of two probes, each probe having a horizontally arranged rod-like core of magnetisable material, the cores in respective probes including an angle of 90.degree. relative to each other and each supporting a winding, means for applying an alternating auxilliary voltage to said windings to alternately magnetize said cores up to saturation, said auxilliary voltage being combined with voltages induced in said windings due to a constant magnetic field composed of an earth's magnetic field component and an interference magnetic field component, said combined voltages producing an alternating measuring voltage, and means for evaluating the duty cycle ratio of positive and negative halfwaves of said measuring voltage, the method comprising the steps of measuring the magnitude and direction (Hg1) of the constant magnetic field in a first position of the vehicle, then measuring the magnitude and direction (Hg2) of the constant magnetic field in a second position in which the vehicle is turned around a vertical axis by 180.degree., then computing from the determined values (Hg1, Hg2) the magnitude of the interference field component (Hs) according to vector equation EQU H.sub.s =1/2(Hg1+Hg2) and storing the computed H.sub.s value for further processing.