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\ 
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 i4<th, 1825. 
Read May 5, 1825. 
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 dril- 
ling, 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, w^ould 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 expe- 
riments. 
