An apparatus provides for controlling molecular clusters in fluid in which an electric or a magnetic energy field is applied to a fluid transversely to the flow direction thereof passing through a pipe means. The width of the energy field parallel to the direction of the fluid flow is less than 1 millimeter. The energy field may be applied by two electrodes or two magnetic ploes in which the overall thickness of the electrodes or magnetic poles is less than 10 millimeters. The electrodes or magnetic poles are preferably provided on the outer peripheral surface of the pipe means.