Pickup Coil Spacing Effects on Eddy Current

Differential probes have existed for many years and have been produced in a variety of configurations. The common feature of these diverse probes is that they detect an ac magnetic field gradient, but it is the desired direction of gradient detection that results in the numerous design variations [1 ,2,3]. The probes commonly used in a production shop for nondestructive testing (NDT) are invariably wound on ferrite cores because of the increased sensitivity that results from the use of high magnetic permeability core materials. However, recent advances in the theory of flawfield interactions have stimulated interest in the use of air core probes [4,5]. The use of air core coils in the detector helps to minimize the complexity of the calculations and leaves the experimenter with very adequate tools for verification studies. This theoretical work has been a critical element in the development of quantitative eddy current measurements.