[ 447 ] '•'' '■ ^ 



LXVIII. Account of Experiments demonstrating a limit to the 

 Magnetizability of Iron. By J. P. Joule, F.R.S. 



[Concluded from p. 315.] fjt*t\/^^' 



On Electro-Magnetic Forces^, 



I HAVE shown in a previous paper, that when a current of 

 voltaic electricity is transmitted through the coils of two 

 electro-magnets, their mutual attraction is proportional to the 

 square of the quantity of electrical force, and also that the lifting 

 power of the horseshoe electro-magnet is governed by the same 

 law. 



I have recently made experiments which prove that the attrac- 

 tion of an electro-magnet, for a magnet of constant force, varies 

 in the simple direct ratio of the quantity of electricity passing 

 through the coil of the electro-magnet. In order to succeed it 

 was necessary to guard against the eifects of induction by a 

 proper arrangement of the apparatus. 



Magnetism appears therefore to be excited in soft iron in pro- 

 portion to the intensity of the magnetizing electrical force ; and 

 electro-magnetic attraction, as well as the attraction of steel 

 magnets, may be considered as proportional to the product of the 

 intensities of magnetic force in the bodies attracting one another. 



I have recently learned that the Russian philosophers, Jacobi 

 and Lenz, have arrived at some of the same conclusions with 

 regard to the laws of electro-magnetic attraction. 



The accompanying figure will perhaps illustrate, with 

 some degree of accuracy, the complex action of the 

 forces which constitute the aggregate attraction which 

 exists between two magnets. The magnetic particles, of 

 which six only, viz. a b cdef, are represented, may be 

 conceived to be of an indefinitely large number spread 

 throughout the region of the poles : the straight lines 

 drawn between the particles represent the directions of 

 the several attractive forces. 



If this view be correct, it is obvious that the closer 

 the approximation of the magnetic particles in each 

 magnet, the greater will be the aggregate attraction ; 

 for in that case the particle «, for instance, will be nearer the 

 particle/, and tlie force exerted between them will be in a less 

 oblique direction. 



It was in consequence of the entertainment of a different 

 hypothesis that I was led, by the experiments recorded in the 

 previous paper, to imagine that I had detected a decrease of 

 power due to an increase of the length of the electro-magnets. 

 I gave a comparison of the attractions of electro-magnets 14 

 inches long and of the several sections yyths, yyths, and y^jths of 

 * Annals of Electricity, vol, iv. p. 474, 



