Abstract: Apparatus for treating liquid to prevent and/or remove scale deposits, made up of at least one solenoid-type electrical coil (5) for generating a magnetic field for acting on the liquid to be treated and an electrical energizing unit (2) having at least one signal output (3, 4) for connecting the ends of the at least one coil (5). The energizing unit (2) is provided with means for generating an energizing signal to be varied in frequency. The at least one coil (5) can also be sited alongside a pipe (1) instead of around a pipe (1), if necessary together with the energizing unit (2) in one and the same housing. To vary the magnetic field generated as a function of the flow rate of the liquid to be treated, the energizing unit is provided with a control input (6, 7) for connecting a flow rate transducer (8).