Abstract: An effective, efficient apparatus for treating a fluid by electron emission or corona discharge having a prolonged useful life can be constructed so as to include a first electrode, a composite dielectric structure located along the side of and spaced from the first electrode so as to define a gap between the first electrode and the dielectric structure and a second electrode. In such a structure preferably the gap is closed off by a flexible seal permitting the thickness of the gap to be adjusted; preferably this thickness is related to the operating parameters of the equipment used to power the apparatus and to the characteristics of the product or products produced in the apparatus. Preferably the dielectric structure consists of at least one layer of a first dielectric material including overlapping flat plate-like particles of an inert dielectric material located directly against the second electrode.