190 Dr. C. Fromme on the Magnetism of Steel Bars. 



the mechanician Apel, and, secondly, after being subsequently 

 freed from their residual magnetism by incandescence. 



The axes were measured by the comparator, the volumes 

 determined by weighing in water. A comparison of the volumes 

 so obtained with those calculated from the lengths of the axes 

 showed a very good agreement of the corresponding values as 

 a proof of the accurate manufacture of the ellipsoid. 



As the greatest difference of the specific gravities in sort I. 

 amounted to 0*0078 and 0*0022 respectively, and in sort II. 

 to 0*0022 and 0*0027 respectively, the homogeneousness is so 

 great that we may beforehand regard as possible the compara- 

 bility of the results obtained with the individual bars of one 

 sort. 



§ 3. Weber's theory of magnetization, having for its basis 

 the notion of rotatable molecular magnets, refers, as is well 

 known, only to soft iron ; in order to find its application to 

 steel also, it needs a modification which shall take into account 

 the phenomenon of residual magnetism. 



Such a modification Maxwell * believes he has discovered in 

 the assumption that the molecular magnets, if rotated less than 

 a definite angle /3 from their position of equilibrium by a mag- 

 netizing force, return to it again after the action of the force 

 has ceased, but that if the angle of rotation (B is greater than 

 /3 , they receive a permanent deviation amounting to an angle 

 /3-/3 . 



No experimental confirmation of this theory has, to my 

 knowledge, followed; probably none has been attempted. 

 Rowland, in his paper " On Magnetic Permeability " &c. (Phil. 

 Mag. 1873, vol. xlvi. p. 155), merely says that his Tables seem 

 to prove that, with weak magnetizing forces, nearly or quite 

 all the magnetism of a bar is temporary. 



Now we have in the two components of the earth's magne- 

 tism very weak magnetizing forces, which, if Maxwell's theory 

 has any significance, must satisfy the requirement of producing 

 no residual magnetism ; and as for the investigation of the 

 magnetisms excited by the terrestrial force we are in possession 

 of a very delicate method, proposed and employed by Weber t, 

 and first applied more extensively by Riecke % to the investi- 

 gation of the magnetization-relations of soft iron, I have com- 



* Treatise on Electricity and Magnetism, vol. ii. p. 79. 



t In the sixth volume of the Abhandl. d. Gesellsch. d. Wissensch. in 

 Gottingen, a Bestimmung der rechtwinkligen Componenten der erdmag- 

 netischen Kraft in Gottingen in dem Zeitraum von 1834-1853." 



\ H Beitrage zur Kenntniss der Magnetisirung des weichen Eisens," 

 Pogg. Ann. vol. cxlix. 



