A short review of scale prevention with magnetic water treatment devices is given. Some explanations for the observed scale preventing effect are discussed and a new hypothesis is presented. We believe that magnetic Lorentz force activates dislocations on the surfaces of seed crystals of impurities in the solution, which increases the precipitation in the bulk solution rather than on the surface of the heating apparatus. Theoretical and experimental investigations were performed with a laboratory model magnetic device which has alternative wound up coils on the joint cylindrical kernel. Theoretical aspects of dimensioning for any industrial water flow rate in a wide range from 0.01 to 1 m3 s-1 are given.