Abstract: An electric precipitator has a plurality of parallel, longitudinally straight and throughoing, and transversely spaced longitudinal main beams, respective parallel, planar, and transversely spaced longitudinal main plates suspended from the beams, a plurality of parallel and longitudinally spaced cross plates extending generally orthogonally between the main plates and defining corners therewith, and respective oblique webs in the corners and each extending at about 45.degree. from the respective cross plate to the respective main plate. Thus the plates and webs together define octagonal-section passages. Respective electrodes extend centrally in the cells and, due to the octagonal section of these cells, there are no dead corners and, in fact, charge concentration is more uniform than in the hexagonal-section systems.