Magnetism of Iron Exhibited by Rotation. 17 



to it, and the former being neutralized as above stated, the 

 new force at right angles to the axis exhibits itself to the 

 observer, by the direction which it impresses on the needle, 

 and which, in all cases, appears to be consistent with this view 

 of the subject. 



We have already stated, that, prior to the publication of the 

 preceding experiments, M. Arago had discovered the curious 

 fact, that if a copper plate be put in rapid rotation under a 

 horizontal needle freely suspended, the rotation of the copper 

 will first deflect the needle out of its true direction, and if the 

 motion be made sufficiently rapid, and the needle and plate 

 of sufficient dimensions, the former will increase in its deflec- 

 tion, till its passage to the opposite point, after which, it will 

 continue to rotate, and ultimately with such velocity, as to be 

 nearly undistinguishable by the eye. 



This experiment was repeated in London by M. Gay 

 Lussac in March or April, and soon after Messrs Herschel and 

 Babbage undertook the experiments of which we now propose 

 to give a sketch. 



In order to obtain more measurable effects than could be 

 arrived at by merely putting the needle in rotation, it was 

 found necessary to reverse the experiment, by setting in ro- 

 tation a strong horse-shoe magnet, and suspending over it 

 thin plates of various metals ; and by preserving always the 

 same speed in the revolving axis, the effect produced was 

 measured by the number of revolutions which the different 

 plates made in a given time. The substances in which signs 

 of magnetism were thus developed, were copper, zinc, silver, 

 tin, lead, antimony, mercury, gold, bismuth, and carbon, in 

 that peculiar metalloidal state in which it is precipitated from 

 carburetted hydrogen in gas works. In the case of mercury, 

 the total absence of iron was secured. 



The relation in which the different metals stand towards 

 each in their magnetic power, was estimated by having thick 

 circular plates of the more usual of them cast all in the same 

 mould, about a foot in diameter, and these being made to re- 

 volve at such a distance below a delicate compass as not to 

 produce rotation, but merely a deviation in the needle, the 

 respective powers were estimated by the deflection they pro- 



VOL. IV. NO. It JAN. 1826. B 



