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Induction by moving the circuits faraday found that

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Induction by Moving the Circuits Faraday found that, by moving the primary circuit toward the secondary circuit, current could be induced in the secondary current in a direction opposite to the primary current. Similarly, Faraday found that moving the secondary circuit toward the primary induces a current opposite to the primary current. Also, moving the secondary circuit away from the primary induces a current in the same direction as the primary current. Maxwell explains that “the direction of the secondary current is such that the mechanical action between the two conductors is opposite to the direction of motion, being a repulsion when the wires are approaching, and an attraction when they are receding.” 6 This electromotive force was observed by Faraday but was given more systematic treatment by H.F. Lenz (see below). Three principles are implied by the concept of induction by motion of the primary circuit. The first is that polarized and directional secondary currents can be induced by moving a straight line primary current over a conducting test surface. Secondly, alternating current could be induced in a conducting secondary circuit or test material when a constant current primary coil is moved cyclically up and down or side to side over a secondary coil or conducting test surface. A third concept implied by the technique of induction from a moving primary circuit would be that of using direct current magnetic field detectors to measure the magnitude of secondary current or eddy currents in a conducting material, under or lagging behind the moving primary coil. A practical example of testing by moving the secondary circuit would be the rapid movement of conductive test material, such as sheet metal in a rolling mill, past a stationary direct current test coil, inducing a flow of current in material both approaching and leaving the area of this local magnetization. Detectors of the eddy current field in 31 History of Electromagnetic Testing M OVIE . Electromagnetic induction.
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either location can respond to local discontinuities or material property variations that influence the amplitude and distribution of the eddy currents. Faraday also found that current could be induced by the relative motion of a magnet and the secondary circuit. Maxwell explains that “if we substitute for the primary circuit a magnetic shell, whose edge coincides with the circuit, whose strength is numerically equal to that of the current in the circuit, and whose austral face corresponds to the positive face of the circuit, then the phenomena produced by the relative motion of this shell and the secondary circuit are the same as those observed in the case of the primary circuit.” 6 The coil of the preceding examples can be replaced by a permanent magnet when relative motion exists between the magnet and test material in eddy current tests, provided that adequate secondary current magnitude and speed of motion can be attained.
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  • Fall '19
  • Magnetic Field, James Clerk Maxwell, Nondestructive testing, History of Electromagnetic Testing

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