Abstract: An eddy current crack detection system has an inductive probe coil forming part of an oscillatory circuit. The coil voltage is fed to a measuring device a change in amplitude of the coil voltage being used as an indication of a crack. In order to compensate for lift-off in the amplitude of the coil voltage, a control signal is generated related to the magnitude of lift-off, and controls the value of a variable damping resistor in the oscillatory circuit.

Abstract: A reflection type probe for an eddy current testing instrument comprises a ferromagnetic core 2, 3 and a driver coil system 8 and a pick-up coil system 9 mounted on the core, the core having four limbs 4 to 7 each with a free end. One of the coil systems 9 is wound on the four limbs with its turn equally distributed between the four limbs and the other coil system 8 is wound on a core part remote from the turns of the coil system 9 and which interconnects the core limbs at least in pairs. Advantageously the coil system which is wound on the limbs 4 to 7 is the pick-up coil system. A detector coil section of the pick-up coil system can then have its turns equally distributed between one pair of limbs and a balancing coil section having its turns equally distributed between the other pair of limbs.