“…According to Lentz’s rule, the secondary magnetic field is directed opposite to the primary field and thus causes a change in the impedance of the coil. This property makes it possible to detect defects in conductive objects [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ] and determine their geometric dimensions [ 20 , 21 , 22 , 23 , 24 , 25 ], magnetic permeability [ 26 , 27 , 28 ], or electrical conductivity [ 29 , 30 , 31 ]. The changes in the impedance components of the sensor, and therefore the sensitivity of the measurement, strongly depends on the frequency of the current that excites the eddy currents and on the geometry of the sensor.…”