Bodies are rej^elled from the Poles of a Magnet. 171 



netic pole acts on an opposite one of constant power, the attrac- 

 tion is proportional to the strength of the former. 



Let us now take a body whose magnetization varies with that 

 of the magnet : a south pole of the strength M induces in such 

 a body a north pole of the strength M', and the attraction which 

 results from then- mutual action is 



MM'. 



Let the strength of the influencing south pole increase from 

 M to nM ; then, assuming the magnetism of the body under 

 influence to increase in the same ratio, the strength of the above- 

 mentioned north pole will become nW, and the attraction, 

 expressed by the product of both, will be 



n^MM'; 



that is to say, the attraction of a body magnetized by influence, 

 and whose magnetism varies as the strength of the influencing 

 magnet, is proportional to the square of the strength of the latter. 



Here then is a mark of distinction between those bodies which 

 have their power of exhibiting magnetic phsenoraena conferred 

 upon them by the magnet, and those whose actions are dependent 

 upon some constant property of the mass : in the latter case 

 the resultant action will be simply proportional to the strength 

 of the magnet, while in the former case a diff'erent law of action 

 will be observed*. 



The examination of this point lies at the very foundation o f 

 our inquiries into the nature of the diamagnetic force. Is the 

 repulsion of diamagnetic bodies dependent merely on the mass 

 considered as ordinary matter, or is it due to some condition 

 impressed upon the mass by the influencing magnet ? This 

 question admits of the most complete answer either by comparing 

 the increase of repulsion with the increase of power in the mag- 

 net which produces the repulsion, or by comparing the attraction 

 of a paramagnetic body, which we know to be thrown into an 

 unusual condition, with the repulsion of a diamagnetic body, 

 whose condition we would ascertain. 



• Bars of iron and bismuth, of the same dimensions, were 

 submitted to the action of an electro-magnet, which was caused 

 gradually to increase in power; commencing with an excitation 

 by one cell, and proceeding up to an excitation by ten or fifteen. 

 The strength of the current was in each case accurately measui'ed 

 by a tangent galvanometer. The bismuth bar was suspended 

 between the two flat poles, and, when the magnet was excited, 



* This test was first pointed out in a paper on the Polarity of Bismuth, 

 Phil. Mag. Nov. 1851, p. '6'i'6. I have reason, however, to know that the 

 fsame thought occurred to M. Poggeudorff previous to the publication oiF 

 rny paper. — J. T, 



N2 



