EXPLICATION OF ARAGO’S MAGNETIC PHENOMENA. 
147 
suspended in such a way that the latter may rotate in a plane parallel to that 
of the former, the magnet tends to follow the motion of the plate ; or if the 
magnet be revolved, the plate tends to follow its motion ; and the effect is so 
powerful, that magnets or plates of many pounds weight may be thus carried 
round. If the magnet and plate be at rest relative to each other, not the 
slightest effect, attractive or repulsive, or of any kind, can be observed between 
them (62.). This is the phenomenon discovered by M. Arago; and he states 
that the effect takes place not only with all metals, hut with solids, liquids, 
and even gases, i. e. with all substances (130.). 
82. Mr. Babbage and Sir John Herschel, on conjointly repeating the ex- 
periments in this country *, could obtain the effects only with the metals, and 
with carbon in a peculiar state (from gas retorts), i. e. only with excellent con- 
ductors of electricity. They refer the effect to magnetism induced in the 
plate by the magnet ; the pole of the latter causing an opposite pole in the 
nearest part of the plate, and round this a more diffuse polarity of its own kind 
(120.). The essential circumstance in producing the rotation of the suspended 
magnet is, that the substance revolving below it shall acquire and lose its mag- 
netism in a finite time, and not instantly (124.). This theory refers the effect 
to an attractive force, and is not agreed to by the discoverer, M. Arago, nor 
by M. Ampere, who quote against it the absence of all attraction when the 
magnet and metal are at rest (62. 126.), although the induced magnetism 
should still remain ; and who, from experiments made with a long dipping 
needle, conceive the action to be always repulsive (125.). 
83. Upon obtaining electricity from magnets by the means already described 
(36. 46.), I hoped to make the experiment of M. Arago a new source of elec- 
tricity; and did not despair, by reference to terrestrial magneto-electric in- 
duction, of being able to construct a new electrical machine. Thus stimulated, 
numerous experiments were made with the magnet of the Royal Society at 
Mr. Christie’s house, in all of which I had the advantage of his assistance. . 
As many of these were in the course of the investigation superseded by more 
perfect arrangements, I shall consider myself at liberty to rearrange them in a 
manner calculated to convey most readily what appears to me to be a correct 
view of the nature of the phenomena. 
* Philosophical Transactions, 1825. p. 467. 
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