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Dear Sir, It is more than two years since, in a conversation I had with you on subjects connected with magnetism, you enquired what effect I thought might result from giving to an iron ball a rapid rotation? The subject however dropped, and it did not occur to me again, till in some speculative views in which I was lately engaged, as to the cause of the rotation of the earth's magnetic poles, the apparent irregularity of the terrestrial directive powers, &amp;c. I was led to consider that, probably, rotation might have a certain influence. We know that iron is rendered magnetic by various processes, as drilling, hammering, &amp;c. and it was possible also by rotation; your query now occurred to my mind; and knowing at the same time that Mr. Christie had found a permanent change in the magnetic state of an iron plate by a mere change of position on its axis, it seemed highly probable this change, due only to a simple inversion, would be increased by a rapid rotation. In this respect, however, I was deceived; for I found afterwards, that all the effect that was produced was merely temporary; and if any permanent change did take place, it was too small in my cast iron shell to be observed with the small compass I employed in these experiments. Being however thus urged to the inquiry, as well by my own speculative views as by your query, and encouraged by Mr. Christie’s results, I resolved to put the idea to the test of experiment, and to attempt it at once upon a scale that should decide the question in the first instance.

philosophical transactions of the royal society of london

XIV. On the temporary magnetic effect induced in iron bodies by rotation. In a letter to J. F. W. Herschel, Esq. Sec. R. S. by Peter Barlow, F. R. S. Communicated April 14th, 1825