354 Mr. H. A. Rowland on Magnetic Distribution . 



r — 



05. 



•10. 



15. 



•20. 



•30. 



00 . 



x = 



101 



8-96 



8-31 



7-89 



739 



600 



By experiment we find that the neutral point is, in all the 

 cases we have given in Table X., between 7*5 and 8*1 inches, 

 which are quite near the points indicated by theory for the proper 

 values of r, though we might expect curve D to pass through the 

 point # = 9, except for the disturbing causes we have all along 

 considered. 



Our formulae, then, express the general facts of the distri- 

 bution in this case with considerable accuracy. 



These experiments and calculations show the change in dis- 

 tribution in an electromagnet when we place a piece of iron 

 against one pole only. In an ordinary straight electromagnet 

 the neutral point is at the centre. When a paramagnetic sub- 

 stance is placed against or near one end, the neutral point 

 moves toward it; but if the substance is diamagnetic it moves 

 from it. 



The same thing will happen, though in a less degree, in the 

 case of a steel magnet ; so that its neutral point depends on ex- 

 ternal conditions as well as on internal. 



We now come to practically the most interesting case of dis- 

 tribution, namely that of a straight bar magnetized longitudi- 

 nally either by a helix around it, or by placing it in a magnetic 

 field parallel to the lines of force ; we shall also see that this is 

 the case of a steel magnet magnetized permanently. This case 

 is the one considered by Biot (Traite de Phys. tome iii. p. 77) 

 and Green (Mathematical Papers of the late George Green, 

 p. Ill, or Maxwell's 'Treatise/ art. 439), though they apply 

 their formula? more particularly to the case of steel magnets. 

 Biot obtained his formula from the analogy of the magnet to a 

 Zamboni pile or a tourmaline electrified by heat. Green ob- 

 tained his for the case of a very long rod placed in a mag- 

 netic field parallel to the lines of force, and, in obtaining it, used 

 a series of mathematical approximations whose physical meaning 

 it is almost impossible to follow. Prof. Maxwell has criticised 

 his method in the following terms (' Treatise/ art. 439) : — 

 " Though some of the steps of this investigation are not rigo- 

 rous, it is probable that the result represents roughly the actual 

 magnetization in this most important case." From the theory 

 which I have given in the first part of this paper we can deduce 

 the physical meaning of Green's approximations; and these are 

 included in the hypotheses there given, seeing that, when my 



