TO CONTROUL THE ATTRACTIVE FORCE OF A MAGNET. 
50 5 
screen : it therefore follows in this instance, that whilst an inconsiderable 
portion of the magnetic action is neutralized, a considerable mass of the screen 
is pervaded ; at the same time a very thin stratum only of the magnet is pene- 
trated b (8). 
11. The diminished influence of a magnet on a metallic disc, observed to 
ensue on intersecting the surface of the disc by radiating grooves varying in 
depth *, may possibly depend on the above-mentioned circumstance (10) ; for 
in this case we actually take away a portion of the substance in which the 
magnetic development takes place, and thus diminish the force. I wish, how- 
ever, to be understood as speaking with some degree of reserve on this point, 
although the conclusion is by no means unwarranted, as in the following- 
experiment. 
( h ). The number of vibrations of a delicately suspended bar in a given arc 
taken in vacuo, in two similar rings of copper of equal weights and quality as 
nearly as may be, did not materially differ, although one of the rings was 
made up of separate concentric laminse, the other being a perfectly solid mass ; 
whereas the removal of a very thin lamina externally from the former caused 
a very decided change in the number of the vibrations of the bar -f-. 
12. The preceding inquiries appear calculated to modify in some measure 
our views concerning the operation of a magnet, which by experiment ( e ) is 
rather the patient than the agent in the production of the observed effects : it 
cannot therefore be considered as a purely active force, much less can it be 
viewed as a substance, from which emanations of an unknown subtile fluid 
are constantly proceeding ; for it may be shown, Exp. (c), that a magnetic 
lamina of steel, supposed without sensible thickness, cannot act at the same 
time on two masses of iron, in every respect alike, when placed between them, 
and at an equal distance from each ; as in this case we should have an annihi- 
lation of power as regards the magnet : hence each mass of iron, if at the same 
time drawn by some other force, and free to move, would drop away from the 
magnetized steel in opposite directions. If therefore the attractive energy of 
a magnet be supposed to arise out of any subtle principle emanating from it, 
* Philosophical Transactions for 1825, p.481. 
f These vibrations were determined in the way described in the Royal Society’s Transactions for 
1831, Part I., p. 76. 
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