MAGNETISM. 



93 



\vill be deflected in a direction contrary 

 to the direction in which that edge of 

 the plate which is nearest to it moves. 

 The deviations of the horizontal needle 

 may be easily deduced from this law, by 

 reference to the motions of this imagi- 

 nary dipping-needle, for they will be 

 such as tend to bring them into the 

 same vertical plane with the latter, 

 which is the situation in which it makes 

 the nearest approach to its line of direc- 

 tion. 



(357.) The investigation of this cu- 

 rious subject was further prosecuted by 

 Mr. Babbage and Mr. Herschel, who, 

 in conjunction, undertook to verify M. 

 Arago's experiments*. After a few 

 trials they succeeded in causing a com- 

 pass to deviate from the magnetic me- 

 ridian, and finally to revolve, by placing 

 under it plates of copper, zinc, or lead, 

 which were put into very rapid rotation. 

 In order to obtain more visible and re- 

 gular effects, however, they found it 

 necessary to reverse the experiment, by 

 setting in rotation a powerful horse-shoe 

 magnet with its poles uppermost, the 

 line joining them being horizontal, and 

 its axis of symmetry being placed verti- 

 cally ; while a circular disc of the sub- 

 stance to be examined was suspended 

 over this magnet. The disc was found 

 to follow the motion of the magnet with 

 various degrees of readiness, according 

 to the substance of which it was made. 

 They obtained in this way signs of 

 magnetic susceptibility from copper, 

 zinc, silver, tin, lead, antimony, mercury, 

 gold, bismuth, and carbon, in that pe- 

 culiar metalloid state in which it is 

 precipitated from carbonated hydrogen 

 in gas-works. Great care was taken, in 

 the case of mercury, to secure the ex- 

 clusion of iron. In other bodies which 

 were tried, such as sulphuric acid, 

 rosin, glass, and other non-conductors, 

 or imperfect conductors, of electricity, 

 rio positive evidence of magnetism was 

 obtained. 



(358.) They next endeavoured to de- 

 termine the comparative intensities of 

 action of these different bodies. Two 

 methods were used for this purpose ; 

 first, by observing the deviation of the 

 compass over revolving plates of great 

 size, cast to one pattern ; and, secondly, 

 by the times of rotation of a neutralized 

 system of magnets suspended over 

 them ; and it is remarkable, that the 

 places of zinc and copper in the scale, 



Philosophical Transactions for 1825, p. 407, 



according as the one or the other of 

 these two methods was employed, were 

 the reverse of each other ; although the 

 same order was assigned to all the other 

 bodies by both methods. 



(359.) On trying the effect of the in- 

 terposition of different bodies as screens, 

 in cutting off or modifying the influence 

 of the rotating bodies, they could not 

 detect any interceptive power, except, 

 as might be expected, in the case of an 

 iron plate, which, when of sufficient 

 thickness, completely destroyed all per- 

 ceptible effect from rotation. 



(360.) The magnetic energy deve- 

 loped by rotation was found to be much 

 diminished by any interruption of con- 

 tinuity in the plate which was acted 

 upon. This fact had previously been 

 noticed by Arago ; but the experiments 

 of Mr. Babbage and Mr. Herschel have 

 verified it in more detail, and have added 

 the curious circumstance, that re-esta- 

 blishing the metallic contact with other 

 metals, restores, in a great measure, the 

 force which had been lost by the divi- 

 sion of the substances ; and this hap- 

 pens even when the metal used for sol- 

 dering has, of itself, but a very feeble 

 magnetic power ; thus affording a means 

 of magnifying weak degrees of magnet- 

 ism. The reduction of the metals to 

 filings, or to powder, was found to pro- 

 duce a still more remarkable diminution 

 of their magnetic energy. The law of 

 diminution of force by increase of dis- 

 tance was next investigated ; but it 

 appeared to follow no constant progres- 

 sion according to any fixed power of 

 the distance, but to vary between the 

 square and the cube. 



(361.) The explanation of these cu- 

 rious phenomena has been attempted 

 on the following principle namely, that 

 in the induction of magnetism, time en- 

 ters as a necessary element ; or, in other 

 words, that a certain appreciable time 

 is required both for the acquisition of 

 magnetic polarity, communicated by 

 induction from a "magnetized body, and 

 for its loss, when the body in which it 

 has been induced returns to the neutral 

 state by the subtraction of all extraneous 

 influence. 



(362.) In order to trace the operation of 

 this principle, let us conceive the north 

 pole of a magnet to move horizontally, at 

 a little distance above a plate of metal, 

 or other substance, having a very low de 

 gree of magnetic susceptibility, and also 

 a very low degree of retentive power. 

 The points over which it passes in sue- 



