A sensor for measuring particles in contaminated media contains an optically transparent measuring cell, through which a capillary tube for conducting and measuring a medium passes, an optical detector, and a magnetic detector. The measuring cell is cylindrical, the capillary tube extends coaxially in the measuring cell and an inspection space is arranged in a center of the measuring cell. On the inspection space an optical lens system having a light source and at least one optical detector is focused. Differential coils for producing and detecting a coaxial alternating magnetic field form the magnetic detector on the shaft of the measuring cell, which differential coils are controlled in such a way that the alternating magnetic field is canceled out in the inspection space.