154 Mr. H. A. Rowland on Magnetic Permeability, 



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 carve is parallel to the axis of Q. The 

 concavity of the curve at its commencement, which indicates a 

 rapid increase of permeability, has been noticed by several phy- 

 sicists, 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*. 

 M. Miiller has given an equation of the form 



3 m 



r==220if|an. 0000 g # 



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. Cazinf; 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 varia- 

 tion 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 mag- 

 netizing-force is applied, an equation which would make the first 

 part of the curve coincide with the dotted line in Plate II. J ; and 

 Maxwell, by addition to the theory, has obtained an equation 

 which replaces the first part of the curve by the broken line§. I 

 believe that Ihave obtained at the least a very close approximation 

 to the true equation of the curve, and will show further on that 

 Q and M must satisfy the equation 



M =Asin V 1) / ( 10 > 



It is very probable that Weber's theory may be so modified 

 as to give a similar equation. 



Space will not permit me to discuss the curves of temporary 



* On the Magnetizing-Function of Soft Iron, especially with the 

 weaker decomposing powers. By Dr. A. Stoletow, of the University of 

 Moscow. Translated in the Phil. Mag. January 1873. See particularly 

 p. 43. 



t Annates de Chimie et de Physique, February 18/3, p. 182. 



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

 having made some mistake in the integration. 



§ Treatise on Electricity and Magnetism, Maxwell, vol. ii. chap. vi. 



