to provide an oscillator circuit which provides a single frequency output or a range of frequencies beyond the above range for application to the coil appropriate frequency selection is made in accordance with the nature of the cell growth and or the quality of water flowing through the pipe in use and as shown in fig3 the apparatus of the invention may be suitably applied to an installation 27 representing a swimming pool the former 12 carrying the coil 13 is located about the pipe 12 and is placed in the pool below the surface of the water the signal generator 14 housed in the housing 15 is mounted in any suitable location and connected to the coil 13 through the wires 32 with respect to fig4 of the drawings and in accordance with a further aspect of the present invention a signal is primarily generated from a single ship microcontroller 22 which in this embodiment comprises a type pic 16c73a microcontroller the 4 00 mhz crystal x 1 and two 15 pf capacitors c 17 and c 18 form the base frequency oscillator for the microcontroller c 14 and c 15 serve as by pass capacitors that stabilize the power supply to the microcontroller the ds1233 10 reset unit ensures the microcontroller starts successfully on every power up the microcontroller generates an internal square wave signal at 10 hz modulated at 7 khz producing a signal with an average frequency of.