Abstract: System for treating fluids, especially water, in a pipe to resist scale deposits from building up in the pipe, includes a field generating coil (10) mounted exteriorly of the pipe, an electronic circuit (14) for energizing the coil to develop an alternating electromagnetic field in the fluid, a sensor (18) for sensing a parameter related to the functioning of the system and for generating a parameter dependant signal, a feed back circuit for feeding back the parameter dependent signal to the electronic circuit to control the circuit, wherein the sensor comprises a field measuring coil (16) mounted exteriorly of the pipe near the field generating coil, the feed back circuit comprises the electronics which together with the measuring coil acts as a field strength meter.