Mr. H. A. Rowland on Magnetic Distribution. 319 



Q" e is the quantity which is usually taken to represent the dis- 

 tribution of magnetism, being nearly proportional to the " sur- 

 face-density n of magnetism, I shall principally discuss it. 



In the first place, then, this equation (5) shows that the dis- 

 tribution of magnetism in a very elongated electromagnet, and 

 indeed in a steel magnet, does not change when pieces of soft 

 iron bars of the same diameter as the magnet are placed against 

 the poles, provided that equal pieces are applied to both ends; 

 otherwise there is a change. This result would be modified by 

 taking into account the variation of the permeability &c. 



Let us first consider the case where the rod projects out of the 

 end of the helix, as in Tables V., VI., and VII. By giving 

 proper values to the constants, we obtain the results given in the 

 last column of the Tables. The agreement with observation is 

 in most cases very perfect. We also see the same variation of r 

 that we before noticed in the rest of the curves, and we see that 

 it is in just the direction theory would indicate from the change 

 of fl. 



In these Tables we come to a very important subject, and one 

 to which I called attention some years back — namely, the change 

 in the distribution when the magnetizing force varies, and which is 

 due to change of permeability. The following Tables and figures 

 show this extremely well, and are from very long rods with a helix 

 a foot long at their centre, as in the last three Tables. The bar 

 in both these Tables was '19 inch in diameter and 5 feet long. 

 The zero-point was at the centre of the bar and of the helix. 

 The Tables give values of Q' e for the magnetizing forces which 

 appear at the head of each column, and which are the tangents 

 of the angles of deflection of the needles of a tangent-galvano- 

 meter. Table VIII. only gives the part covered by the helix. 

 Both Tables are from the mean of both ends of the bar. 



Table VIII. 





Strength of magnetizing current. 



X. 





•108. 



194. 



•378. 



•600. 







u 



3-2 



7 

 •9 



•6 

 •6 



2 

 3 



J 24 



27 



•9 



•8 



33 



3-9 



17 



•8 



4 

 5 

 6 



40 



60 



40 



3-2 



57 



87 



9-3 



147 



