a method for determining the frequency at which the optimum sor exists for a particular system the object to be detected for example the leadscrew 20 is arranged distally from the sensor 10 for example at its furthest distance from the sensor 10 in the case of the leadscrew the control rod may be fully inserted into the nuclear reactor for example with the leadscrew 20 in this position the primary coils are provided with ac current at a range of two or more discrete frequencies f and the output signal v s from the secondary coils measured and recorded for each respective frequency the result of such an exercise is shown in fig6 for example the object to be detected for example the leadscrew 20 is also arranged at proximally to the sensor 10 for example at its nearest position to the sensor 10 in the case of the leadscrew 20 the control rod may be at its maximum withdrawal from the nuclear reactor for example with the leadscrew in this position the primary coils 12 are provided with ac current at the same range of the same two or more discrete frequencies f and the output signal v s from the secondary coils 14 measured and recorded for each respective frequency the result of this exercise is shown.