JAMIN'S OBSERVATIONS. 405 



only commences when X = L; the maximum M' towards which it tend?, 

 and which is figured by the right line C'D', is equal to 0'8i M. 



It may be remarked that the residual magnetism thus calculated, 

 corresponds to the case in which the magnetisation of the body itself 

 produces only an inappreciable demagnetising force ; these results 

 correspond then only to the case of a very long body magnetised 

 longitudinally. 



It is difficult to admit that a discontinuous curve like that which 

 represents the temporary magnetism can be an exact expression of 

 the phenomenon. Nevertheless, this theory leads to curious con- 

 sequences relative to the successive action of magnetising forces of 

 opposite signs, and which have been verified experimentally. 



Let us suppose that a piece of iron after having been submitted 

 to the action of a force X , has acquired a permanent magnetisation. 

 A new force - X 2 of the same direction is without effect as long as 

 it is less than X , and if it is greater than X the residual magnetism 

 is the same as if the original force had not acted. 



If the new force - X 2 is in the opposite direction, it produces a 

 permanent effect long before it reaches X ; the residual magnetism 

 seems to be destroyed for a certain value of this force, but the 

 metal is not in the neutral state, for it is insensible to the action of a 

 force - X, so long as X is less than X 2 , while a feebler positive force 

 produces a permanent magnetisation in the original direction. 



431. JAMIN'S OBSERVATIONS. Jamin gives a different expla- 

 nation of these phenomena. He assumes that the action of the 

 field on a bar extends to a greater or less depth according to its 

 strength. When the apparent magnetisation has become zero, the 

 magnetism is not destroyed ; it was merely a case of the super- 

 position of two opposite magnetisations. An inverse field of less 

 strength than X 2 has no action on the superficial layer, but a direct 

 field of less strength forms a new superficial magnetisation, the 

 action of which is added to that which was previously there. 



M. Jamin verified these theoretical ideas by removing the super- 

 ficial layer of inverse magnetisation, and exposing the subjacent 

 layer of direct magnetisation. He succeeded in doing this either 

 mechanically, by grinding or filing away the outer surface of the 

 magnet, or chemically by dissolving it with acid. 



It must, however, be observed that this predominance of the 

 surface layers is perhaps an accidental phenomenon peculiar to steel, 

 and simply dependent on the constitution 'of this metal. For, in 

 the case of highly tempered bars, such as those which are sought 

 for the construction of magnets, the tempering is necessarily very 



