C 497 1 
XXII. On the magnetism developed in copper and other substances 
during rotation. In a Letter from Samuel Hunter Christie, 
Esq. M. A, ^c, to J. F. W. Herschel, Esq. Sec, R, S. Com • 
municated by J. F. W. Herschel, Esq, 
Read June i6, 1825. 
Dear Sir, 
A s you inform me that you are drawing up an account of 
your magnetical experiments, I send you a brief account of 
those which I have made : they may possibly bear upon 
some of the points which you have had under consideration ; 
and in this case you will not be displeased at being able to 
compare independent results. 
After having made experiments with a thin copper disk 
suspended over a horse-shoe magnet, similar to those which 
I witnessed at Mr. Babbage's, I made the following. 
A disk of drawing paper was suspended by the finest brass 
wire (No. 37) over the horse-shoe magnet, with a paper screen 
between. A rapid rotation of the magnet (20 to 30 times 
per second ) caused no rotation in the paper, but it occasion- 
ally dipped on the sides, as if attracted by the screen, which 
might be the effect of electricity excited in the screen by the 
friction of the air beneath it. 
A disk of glass was similarly suspended over the magnet : 
no effect produced by the rotation. 
A disk of mica was similarly suspended : no effect. 
The horse-shoe magnet was replaced by two bar magnets, 
each 7.5 inches long, and weighing3 oz, iGdwt. each, placed 
