48 HENUY A. EOWLAND 



The best method of studying these Tables is to plot them: one 

 method of doing this is to take the value of the magnetizing-force as 

 the abscissa, and that of the permeability as the ordinate; this is the 

 method used by Dr. Stoletow; but, besides making the complete curve 

 infinitely long, it forms a very irregular curve, and it is impossible to 

 get the maximum of magnetism from it. Another method is to employ 

 the same abscissas, but to use the magnetism of the bar as ordinates; 

 this gives a regular curve, but has the other two disadvantages of the 

 first method; however, it is often employed, and gives a pretty good 

 idea of the action. In Plate II, I have given a plot of Table V with 

 the addition of the residual or permanent magnetism, which shows the 

 general features of these curves as drawn from any of the Tables. It 

 is observed that the total magnetism of the iron at first increases very 

 fast as the magnetizing-force increases, but afterwards more and more 

 slowly until near the maximum of magnetism, where the curve is 

 parallel to the axis of Q. The concavity of the curve at its commence- 

 ment, which indicates a rapid increase of permeability, has been noticed 

 by several physicists, and was remarked by myself in my experiments of 

 January, 1871; it has now been brought most forcibly before the public 

 by Dr. Stoletow, whose paper refers principally to this point. 17 M. 

 Miiller has given an equation of the form 



to represent this curve; but it fails to give any concavity to the first 

 part of the curve. A formula of the same form has been used by M. 

 Cazin ; 18 but his experiments carry little weight with them, on account 

 of the small variation of the current which he used, this being only 

 about five times, while I have used a variation in many cases of more 

 than three hundred times. 



Weber has obtained, from the theory that the particles of the iron 

 are always magnetic and merely turn round when the magnetizing- 

 force is applied, an equation which would make the first part of the 

 curve coincide with the dotted line in Plate II ; 19 and Maxwell, by addi- 

 tion to the theory, has obtained an equation which replaces the first 



17 On the Magnetizing Function of Soft Iron, especially with the weaker decom- 

 posing powers. By Dr. A. Stoletow, of the University of Moscow. Translated in 

 the Phil. Mag., January, 1873. See particularly p. 43. 



18 Annales de Chimie et de Physique, February 1873, p. 182. 



19 This is according to Maxwell's integration of Weber's equation, Weber having 

 made some mistake in the integration. 



