STUDIES ox MAGNETIC DISTRIBUTION 



109 



quantity which is usually taken to represent the distribution of mag- 

 netism, being nearly proportional to the "surface-density" of mag- 

 netism, I shall principally discuss it. 



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

 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 permea- 

 bility, &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 



TABLE VIII. 



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 p. 



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- 

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

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



