negative electrode 9 is arranged there is an insulator 11 made of an electrically insulating material fig5 also shows diagrammatically how an ignition coil 13 is connected to the spark plug 3 when soot particles are formed during the combustion stroke these adhere to the insulator 11 the more soot particles that are formed the thicker the soot deposit on the insulator 11 becomes the soot particles conduct current as a result of which a leakage current i l arises between the positive electrode 8 and the housing 10 which is connected to earth the size of the leakage current i l is a measure of the soot particle content in the exhaust gases the leakage current affects the measurement of the ionic current i j in such a manner that when soot particles adhere to the insulator 11 the leakage current i l increases and thus the level of the ionic current signal also increases during measurement of the ionic current i j the more soot that adheres to the insulator 11 the more the level of the graph 1 representing the ionic current i j is raised the leakage current i l can itself be measured by a predetermined current being induced between the electrode 8 and the housing 10 the current that will then be transmitted between the electrode 8 and the housing 10 constitutes the leakage current i l fig2 shows this leakage current i l as a straight line at the end of and after the post ionization stage p and also before and during the ignition stage s the leakage current i l is suitably measured.