A varistor includes a varistor element, a first layer which is porous on the varistor element, and a second layer on th first coating. The precast slabs, which incorporate raised, truncated domes on the top side and a floated finish on the bottom side, are made to be set in the frame which has been set and cast into fresh concrete. Detection is carried out in a measuring cell having a working electrode in order to determine the concentration of the marked microparticle. An insulating layer is interposed between the first and second electrodes to facilitate plasma formation via dielectric barrier discharge in the gas passages between the first and second electrodes.