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, &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, &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.
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
