The element is a
two winding device: the first consists of two excitation coils wound
along the length of high permeability wire, positioned side by side
within a bobbin onto which the second coil is wound. These coils
are arranged so that in the absence of an external magnetic field,
no voltage is produced in the secondary coil.
One type of element
(LFG-A13) incorporates a feedback winding that opposes the external
field - using automation, the element can be used as a null indicator.