298 BELL SYSTEM TECHNICAL JOURNAL 



reader who feels that these principles are familiar, or self-evident, or 

 unimportant, may leap to the second section and the third, in which 

 the I-vs-H curves are accepted as the data of experience. 



I shall not venture a definition of ferromagnetism until nearly the 

 end of the article. Such a definition is not easy to make, unless one 

 takes refuge in the statement that "ferromagnetism is the kind of 

 magnetism displayed by iron." I can only regret the frequency with 

 which such ponderous words as ferromagnetism and permeability and 

 susceptibility and magnetization and magnetostriction must needs ap- 

 pear. The subject is encumbered by its heavy vocabulary; it ought 

 to have a new one made up entirely of short and vivid words. 



A, Analysis of the Magnetization of Magnetized Bodies. 



Let us imagine a collection of magnets such as one frequently sees, 

 horseshoe magnets for example, with their ends painted red and blue. 

 We know that (if the painting was done properly) the red end of each 

 attracts the blue ends and repels the red ends of the others; the blue 

 end attracts those which the red end repels and repels those which the 

 red end attracts. It seems as if the ends of the magnets were covered 

 with invisible substances — one kind on all the red ends, the other on 

 all the blue ends — so constituted, that a sample of either substance 

 attracts all samples of the other sort, repels all samples like itself. 

 Coulomb found that if the magnets were long and slender, so that the 

 power of attracting and repelling was concentrated very closely about 

 the extremities of each, these extremities attracted or repelled one 

 another according to an inverse-square law. That suggested gravita- 

 tion and electric force; which suggested in turn that, even as matter is 

 the source of gravitation and electric charge is the source of electric 

 force, so also there is an invisible thing called magnetism which in- 

 habits iron — or rather, two invisible things, positive magnetism and 

 negative magnetism, which may be pulled and pushed around inside 

 and over the surface of a piece of iron. This notion of a pair of in- 

 visible and mobile fluids is very helpful, and I shall use it in several 

 passages; yet the reader must not take it as corresponding to the actual 

 reality. We cannot imagine two or even one perfectly mobile mag- 

 netic fluid, for a well-known reason. 



The reason is, that even though a magnet may appear to carry noth- 

 ing but positive magnetism on one of its ends and nothing but negative 

 magnetism on the other, yet it is not possible to cut off anywhere aj 

 piece containing only one of these kinds. In fact it is not possible 

 anywhere to cut off a piece not containing equal quantities of the two 

 kinds of magnetism. Any piece of matter always contains as much 



