on the insulating layer or are introduced into the insulating layer thus giving rise to a plane surface structure for the best possible measurement of a thin film of soot consequently it is also possible in accordance with the contemplated embodiment shown in the right hand half of fig5 to prevent interruptions in the conductivity by the soot at vertical edges of the electrodes a measurement using a sensor constructed in accordance with fig1 is illustrated in fig6 in the course of the first minute of the measurement the resistance of the sensor rises since desorption occurs during the heating phase to 350 u00b0 c after approximately 3 minutes a regeneration step is performed at 800 u00b0 c here the resistance is reduced by virtue of an intrinsic conductivity of the substrate after the sensor has again reached its operating temperature of 350 u00b0 c after approximately 4 3 minutes the sensor is exposed to the soot of an open diesel flame after a few seconds after ignition the sensor resistance falls steeply over a number of decades and it reaches a stable value while soot is still being formed as a result of the regeneration at 800 u00b0 c over approximately 9.