A Question of Electromagnetic Induction

Try to give a really clear explanation of what is happening here


At the left we see a bar magnet to which a copper disc has been cemented at the North pole. Sliding contacts have been provided at the axis, and at the rim of the copper disk. The magnet is rotated at constant speed in one direction or the other. Faraday performed this experiment in 1831-2 in his experiments on electromagnetic induction. The question is, did he observe a voltage V, or did he not?

Click anywhere on this sentence for the answer.

This is just one of a great many questions that tax one's understanding of electromagnetic induction. Electrical engineering professors used to enjoy thinking them up, and watching their students squirm. Rotating magnets and sliding contacts always provided entertainment. Many professors came up with the wrong answers, however!

Thomas Martin's Faraday's Discovery of Electro-Magnetic Induction (London: Edward Arnold & Co., 1949) is a good short account of Faraday's early experiments, including this one.


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Composed by J. B. Calvert
Created 2 May 2000
Last revised 2 May 2000