188 PROCEEDINGS OF THE AMERICAN ACADEMY. 



disposal two magnetizing solenoids very much longer than any which 

 have ever been used before, as far as he knows. Thus it was made 

 possible to obtain complete series of magnetization curves, yielding 

 tables of values for N, for a large number of iron rods, ranging in 

 diameter from 0.23$ 1 cm. to 1.905 cms. The results disclose quite a 

 marked dependence of N on the D, the L/D and / being considered 

 constant. In fact the general rule may be stated that the value of N 

 decreases as the diameter of the iron rod increases. 



In the work both the "reversal" and the "step-by-step" methods 

 have been used, and the results obtained may be interesting to some 

 who have had occasion to observe the peculiar disagreements in the 

 results given by these two methods. As a rule the iV's calculated 

 from reversal curves will be smaller than those obtained from the 

 "step-by-step" method under the same conditions. 



Introduction. 



When a piece of homogeneous isotropic soft iron of any shape is 

 placed in a magnetic field, it will always become magnetized, and the 

 induced magnetism will in general show its existence by changing the 

 original field outside the iron. The only exceptional cases are those 

 in which the iron is "endless," that is, it is in the form of an anchor 

 ring or a rod of infinite length, with the magnetizing solenoid wound 

 directly over the iron. Whenever an apparent magnetic distribution 

 of superficial charge o- and volume charge p is induced by polarization 

 on or in any body of iron, the magnetic field H^ due to it combines 

 with the magnetizing field H' to give a resultant field H, so that the 

 actual field which determines the intensity of magnetization / is given 

 at every point by the vector equation 



H=H' + Hr, 



and / ■= kH, where k = susceptibility of the iron. Outside the iron 



H will usually be less than H' in some portions of space, and in others 



it will be greater than ff'. But inside the iron H will in general, 



perhaps always, be less than JI'. Thus in the case of a sphere of soft 



iron placed in a uniform field ff', we shall have, from the theory given 



in most of the text-books on electricity and magnetism,*^ a uniform field 



4- 

 of intensity H=H' — — / ^vithin the sphere at any point A, while the 



o 



■f Maxwell, II, §§ 437-438; Webster's Electricity and Magnetism, p. 371; 

 Peirce's Newtonian Potential Function, p. 205. 



