Patent Number: 
Section: claims

1. A method of optimising the output of a sensor for indicating the relative location of a metallic object, the sensor being of the type having a primary electromagnetic coil arranged to generate a time varying magnetic field; and a secondary electromagnetic coil arranged to detect the time varying magnetic field as affected, directly or indirectly, by the object and to output, on the basis of the detected time varying magnetic field, a signal indicative of the relative location of the object, the method including the steps of:supplying the primary coil with an alternating current to result in the generated time varying magnetic field;locating the object in a first position and recording the signal output by the secondary electromagnetic coil for a range of respective frequencies of the supplied alternating current;locating the object in a second position and recording the signal output by the secondary electromagnetic coil for the range of respective frequencies of the supplied alternating current;calculating, for each of the respective frequencies, a value for the span to offset ratio of the measured signals on the basis of the respective signals measured for the object in the first and second positions, the calculating step including, for each respective frequencycalculating the difference between the amplitudes of the signals measured for the object in the first and second positions, anddividing the difference by the amplitude of the signal measured for the object in the second position; anddetermining the frequency of the supplied alternating current which provides the maximum span to offset ratio on the basis of the calculations. 2. A method according to claim 1, whereinwhen the object is in the first position, the output from the secondary coil is a maximum; and/orwhen the object is in the second position, the output from the secondary coil is a minimum. 3. A method according to claim 1, wherein at least one of the primary and secondary electromagnetic coils of the sensor is wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. 4. A method according to claim 1 wherein the primary and secondary coils of the sensor are arranged coaxially. 5. A method according to claim 1, wherein the primary coil of the sensor is wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. 6. A method according to claim 1, wherein the secondary coil of the sensor is wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. 7. A method according to claim 1, wherein the primary and secondary coils of the sensor are each wound about the same core body formed of a material having the same conductivity and/or magnetic permeability as the object. 8. A method according to claim 1, wherein the primary coils of the sensor are mutually arranged in electrical series; and/or wherein the secondary coils of the sensor are separately mutually arranged in electrical series. 9. A method according to claim 1 wherein the primary and/or secondary coils of the sensor are formed of an alloy comprising 86% copper, 12% Manganese and 2% Nickel. 10. A method according to claim 1 wherein a core body of the sensor is formed of a material having the same conductivity and/or magnetic permeability as the object. 11. A method according to claim 1 wherein a core body of the sensor is formed of the same material as the object. 12. A method according to claim 1, wherein the metallic object is attached to a nuclear reactor control rod. 13. A method according to claim 1, wherein the sensor is positioned within a metallic tube and the metallic object is arranged to move relative to the tube between a position of minimum overlap and a position of maximum overlap of the tube and the object. 14. A method according to claim 13, wherein the first position is a position where there is minimum overlap between the tube and the object. 15. A method according to claim 13, wherein the second position is a position where there is maximum overlap between the tube and the object. 16. A method according to claim 1, wherein the sensor includes a plurality of primary electromagnetic coils. 17. A method assembly according to claim 16, wherein the sensor includes a plurality of secondary electromagnetic coils. 18. A method assembly according to claim 17, wherein the plurality of primary and secondary coils are arranged in a mutually alternating sequence of primary and secondary coils.