Variable magnetic coupling transducers

A transducer of this type relies on variable coupling between magnetic coils. An example is a Linear Variable Differential Transformer
(LVDT):

Very common, especially in aircraft systems. It consists of three coils in line, two outer coils acting as the secondary of a transformer and
one central coil acting as the primary. Note that the secondary coils are connected in the opposite directions.

The displacement moves the ferromagnetic rod in relation to the coils and changes coupling between the coils. When the rod is off- center the coupling imbalance causes the AC voltage difference at the output of LDVT