or discharging cycle and changes increases as the filling operation progresses variable modulus divider 74 receives an input from clock 70 as well as from shift register 72 and produces a pair of output frequencies such as 6 25 khz and 5 68 khz one of which represents binary 1 39 s of the signal from the shift register and the other of which represents binary 0 39 s of the same signal the output of divider 74 is passed through a low pass filter 76 to the rotatable coupling coil 12 as signals indicative of the charging capacitance of conductors 7 at this stage of the operation of the monitoring system illustrated in fig1 there have been created audio frequency signals which indicate the changing capacitance of conductors 7 as the filling operation progresses in a like manner similar signals will have been generated by the circuitry of transmitter 21 to indicate the changing capacitance of conductors 18 it is possible to operate with these signals to achieve the desired comparisons and hence a measure of the filling effectiveness in a number of ways the remaining circuitry of fig4 illustrates one arrangement for achieving the desired results the audio.