ET02.pdf

# Body extending from it on all sides their directions

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body extending from it on all sides, their directions being modified by the presence of other bodies. He even speaks of the lines of force belonging to a body as in some sense part of itself, so that in its action on distant bodies it cannot be said to act where it is not. This, however, is not a dominant idea with Faraday. He would probably have said that the field of space is full of lines of force, whose arrangement depends on that of the bodies in the field, and that the mechanical and electrical action on each body is determined by the lines which abut on it.” 6 Maxwell describes the first form of Faraday’s law: “The primary circuit is connected with a voltaic battery by which the primary current may be produced, maintained, stopped, or reversed. The secondary circuit includes a galvanometer,” which is placed so that the primary current does not affect it. Parts of the primary and secondary currents are straight wires placed parallel and near to each other. When a current is suddenly sent through the primary circuit, Maxwell explains, “the galvanometer of the secondary circuit indicates a current in the secondary straight wire in the opposite direction. This is called the induced current. If the primary current is maintained constant, the induced current soon disappears, and the primary current appears to produce no effect on the secondary circuit. If now the primary current is stopped, a secondary current is observed, which is in the same direction as the primary current. Every variation of the primary current produces electromotive force in the secondary circuit. When the primary current increases, the electromotive force is in the opposite direction to the current. When it diminishes, the electromotive force is in the same direction as the current. ... These effects of induction are increased by bringing the two wires nearer together. They are also increased by forming them into two circular or spiral coils placed close together, and still more by placing an iron rod or a bundle of iron wires inside the coils.” 6 This experiment demonstrates the fundamental principles for using magnetizing coils in eddy current testing. The need for a time varying primary current is clearly indicated. The advantage of close coupling or spacing between the magnetizing coil and test metal surface is also shown. This translates into control of liftoff of probe coils and preference for high coil fill factors with encircling coil eddy current tests. The need for pulsating or alternating primary current is also now evident. Finally, the advantages of using ferrite or iron cores in eddy current probe coils are suggested. Eddy current test systems at the beginning of the twenty-first century make full use of each of these principles, enunciated clearly by Faraday in 1831.

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• Fall '19
• Magnetic Field, James Clerk Maxwell, Nondestructive testing, History of Electromagnetic Testing

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