SHUDDEMAGEN. — DEMAGNETIZING FACTORS FOR IRON RODS. 189 



intensity is H — — 1 + ±irl at the point B just outside the sphere on 



that line of ffl which passes through the centre of the sphere, while at 

 all points C just outside the sphere and lying in a plane passing through 

 the centre of the sphere and perpendicular to the H'-line mentioned, 



4r7T 



the intensity will be H' — —I. Figure 1, reproduced from Figure 



76 on page 373 of Webster's " Theory of Electricity and Magnetism," 

 shows the resultant lines of force in this case. For a ring or an 

 infinite rod of constant cross-section with the magnetizing solenoids 

 properly arranged, we should get H, = 0, and H = H. 



Figure 1. 

 A sphere of permeability 3 in a uniform magnetic field. 



At any point in an iron body subjected to a magnetizing field H\ 

 the strength of the field //, can be regarded as a function of /. If in 

 particular we write the scalar equation 



Hi = XI 



and remember that in practical cases the H t is a field opposed to H', 

 or tending to demagnetize the iron, then we may speak of the factor 

 iVas the "demagnetizing factor" of the particular body of iron at the 

 point considered, with reference to the permanent magnetizing field 

 used, which in all practical cases will be a uniform one. Since H t is 

 in general an unknown function of /, therefore N is also some function 

 of I. As the Hi in the cases to be considered will be directed exactly 

 oppositely to H in that part of the iron which we shall be interested 



