200 Prof. E. Bouty's Studies on Magnetism. 



effect of this force to a sort of friction which opposes the separa- 

 tion of the combined magnetic fluids, or their reunion when they 

 are separated. This comparison between two orders of pheno- 

 mena so complex and so little understood as magnetization and 

 friction is necessarily very artificial, and excludes a great num- 

 ber of phenomena furnished by experiment. In fact, the as- 

 sumption of a coercive force analogous to friction would seem 

 to imply the following propositions : — 



1. The coercive force is opposed to any induction-effect on 

 the part of magnetic forces below a certain limit C. 



2. It reduces the effect of aDy force F >C to that which would 

 be produced by a force equal to E — C, if the coercive force did 

 not exist ; and 



3. After the cessation of F it preserves the acquired magnetism 

 up to a limit equal to the magnetism which would be produced 

 by a force C continuing to act in the direction of the force F, if 

 the coercive force had no existence. 



The only mathematician who, to my knowledge, has treated 

 a problem in the hypothesis of the coercive force, Green, does 

 not appear to have formed a different idea of it. 



In order to appreciate the experimental value of the hypo- 

 thesis, we have only to compare the three preceding propositions 

 with three others which we will borrow from a paper by Mr. 

 Rowland*, and which, founded on experiment, agree also per- 

 fectly with all the known facts. 



" 1 . Nearly or quite all the magnetism of a bar is, with weak 

 magnetizing forces, temporary; and this is more apparent in steel 

 than in soft iron. 



"2. The temporary magnetism increases continually with the 

 current. 



"3. The permanent magnetism at first increases very fast with 

 the current, but afterwards diminishes as the current increases, 

 when the iron is near its maximum of magnetism. 33 



It is remarkable that, if the hypothesis of the coercive force 

 is incapable of representing the whole of the phenomena, it 

 nevertheless represents very well the course of the permanent 

 magnetization. We reproduce (fig. 6) a curve drawn by Row- 

 land after his experiments, taking for abscissae the magnetic 

 forces, and for ordinates the corresponding magnetic moments 

 acquired permanently by the unit of volume of a cylinder of in- 

 definite length : it is the curve P Q R S T. We insert also a 

 broken line M N L, which reproduces in general features the 

 course of the curve : this corresponds to the hypothesis of the 

 coercive force by making C = 5 and supposing that the perma- 

 * Phil. Mag. August 1873. 



