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Faraday's Law of magnetic induction states that the e.m.f. acting around a closed circuit is equal to the time rate of change of flux through that circuit. If the closed circuit is a conducting medium, there will flow an electric current which is proportional to the conductivity of the material and the time rate of change of the flux. Electrical currents so induced in a conducting material are often referred to as eddy currents. Eddy currents are thus induced into a conducting material by (1) a time rate of change of the magnetic field through the material, by (2) a relative motion of the conducting material and a magnetic field, or by (3) a combination of (1) and (2). Because the magnitude of the induced eddy currents is dependent upon the resistance of the equivalent conducting path, these currents offer a method of detecting electrical discontinuities in a conducting material. A flaw, such as a season crack in brass, is a definite electrical discontinuity which raises the resistance of the equivalent path of the region.
Dinger, J. E.
Head, Naval Research Laboratory, Washington, D. C.
Supt., Naval Research Laboratory, Washington, D. C.