8 

 NOTES ON MAGNETIC DISTKIBUTION 



[Proceedings of the American Academy of Arts and Sciences, XI, 191, 19^, 187(i. Pre- 

 sented June 9, 1 875] 



In two papers which have recently appeared on this subject, by Mr. 

 Sears (Amer. Jour, of Science, July, 1874), and Mr. Jacques (Proc. 

 Amer. Acad. of Sciences, 1875, p. 445), a method is used for determining 

 magnetic distribution, founded on induced currents, in which results 

 contrary to those published by M. Jamin have been found. It does not 

 seem to have been noticed that the method then used does not give 

 what we ordinarily mean by magnetic distribution. In mathematical 

 language, they have measured the surface integral of magnetic induc- 

 tion across the section of the bar instead of along a given length of its 

 surface. 1 M. Jamin's method gives a result depending on the so-called 

 surface density of the magnetism, which is nearly proportional to the 

 surface integral of the magnetic induction along a given length of the 

 bar. Hence the discrepancy between the different results. Had the 

 experiments of Mr. Sears and Mr. Jacques been made by sliding the 

 helix inch by inch along the bars, their results would have confirmed 

 those of M. Jamin. Four or five years ago, I made a large number of 

 experiments in this way, which I am now rewriting for publication, and 

 where the whole matter will be made clear. At present, I will give the 

 following method of converting one into the other. Let Q be the sur- 

 face integral of magnetic induction across the section of the rod, and 



let Qe be that along one inch of the rod: then Qe <x ^.x beinar the 



(IX 



distance along the rod. Hence, M. Jamin's results depend on the rate 

 of variation of the magnetization of the rod, while those of Mr. Sears 

 and Mr. Jacques depend on the magnetization. In conclusion, let me 

 heartily agree with Mr. Jacques's remarks about M. Jamin's conclusions 

 from his experiments. Such experiments as those give no data what- 

 ever for a physical theory of magnetism, and can all be deduced from 

 the ordinary mathematical theory, which is independent of physical 



1 Maxwell's Electricity and Magnetism, art. 402. 



