field locating object in first position and recording signal output by secondary electromagnetic coil for range of frequencies of supplied alternating current locating object in second position and recording signal output by secondary electromagnetic coil for range of frequencies of supplied alternating current calculating for each of frequencies a value for span to offset ratio of measured signals on basis of respective signals measured for object in first and second positions determining frequency of supplied alternating current which provides maximum span to offset ratio on basis of calculations application_number US 201514734649 A description fig1 shows schematic cross sections of a sensor 10 the schematic only shows half of the full arrangement the full arrangement being mirrored about the dashed line a a to shown in fig1 the sensor 10 comprises a plurality of primary coils 12 coaxially arranged with a plurality of secondary coils 14 one or more primary coils 12 may be provided one or more secondary coils 14 may be provided where a plurality of primary coils 12 are provided the respective primary coils may be connected in electrical series where a plurality of secondary coils 12 are provided the respective secondary coils may be connected in electrical series the primary and secondary coils are arranged in a mutually alternating physical series or sequence such that the.