Abstract: In an apparatus for treating liquid with electromagnetic flux, one or more coils surrounding a liquid conducting pipe are excited by a switching circuit including a triode switch turned on and off in response to the voltage applied to the coil or coils in such a way that during each operative half cycle of the supply voltage a current is conducted through the coils during an initial portion of the half cycle and then at a substantial time before the end of the half cycle, the switch is turned to an open condition causing the current built up in the coils to be taken up by a series resonant circuit, formed by the inductance of the coil or coils an associated given capacitance and the power supply, to produce a ringing current in and a ringing voltage across each coil and consequent ringing electromagnetic flux in the liquid passing through the pipe. The apparatus may have either a half wave or full wave mode of operation.