89 
the particles of magnetic bodies, &c. 
magnetism which takes place on making a circular cut in the 
disc, especially near to the magnets, it would follow, that if a 
copper disc were reduced to a series of concentric rings of 
small breadth, the magnetism developed in it would be 
scarcely appreciable under whatever part the magnets might 
revolve : the same would follow from the experiment of 
Mr. Babbage and Mr. Herschel, if a copper disc, by conti- 
nual cutting in the direction of radii, were reduced nearly to 
a series of very small sectors ; so that if a disc were separated 
in both directions, so as to be reduced to pieces of small 
magnitude, even though not in a state of powder, there can, 
I think, be no doubt that the induced magnetism would be 
rendered insensible. 
The value of u (Table VII.) when the disc was cut, as in 
fig. 4. the rings and small disc being retained, is rather less 
than its value when they were removed : this, however, is only 
to be attributed to the almost unappreciable effect produced 
by the pieces in the interior, and unavoidable errors of obser- 
vation. This will be evident from the results in the following 
table, obtained by placing an entire disc, 4-4 inches in dia- 
meter in the ring, instead of the small disc and rings. The 
observations were made some time after the preceding, when 
the apparatus had been moved, and the disc was not at pre- 
cisely the same distance from the magnets, which was the 
reason of the effects being less than before. The magnets 
revolved at the same distance from the axis as before, 3-2 
inches ; first under the ring, diameters 4-4 inches, and 8-4 
inches, with the disc 4*4 inches in diameter in the interior ; 
then under the ring alone, and afterwards under the disc 
alone. The effect of even a slight current of air was excluded 
mdcccxxvii. N 
