CONTEMPORARY ADVANCES IN PHYSICS 297 



wish to postpone as long as possible the attack upon the intrinsic 

 properties of solids; but there is no evading it in the study of ferro- 

 magnetism, for this is a property of solids only, and not even of trans- 

 parent solids at that. One would wish at least to be permitted to 

 restrict the study to pure elements or simple compounds; but many of 

 the most interesting of the ferromagnetics belong among those be- 

 wildering substances the alloys, which form what the mathematicians 

 describe as a continuum beside and among the great yet finite number 

 of chemical compounds. If one were to work only with perfectly pure 

 iron (supposing that one could get such a substance, or could recognize 

 it when he had it!) the problem would not yet be simple; for every 

 species of mechanical and thermal treatment, and magnetization itself, 

 would transform the iron into a new sort of solid. 



These difficulties I will stress in the pages of this article. There is 

 another. The information about a ferromagnetic substance — the 

 prime material required for theorizing or for practical applications — 

 is usually furnished in the form of so-called I-vs-H curves; that is to 

 say, relations between the "intensity of magnetization" and the 

 "magnetizing field." These curves play the part of the ultimate data 

 of experience. Yet they are not ultimate data; the "magnetizing 

 field" is seldom actually measured, the "intensity of magnetization" 

 almost never. These entities / and // are deduced from experience 

 by means of a theory. The theory is indispensable. If an unin- 

 structed person were presented with a number of variously-shaped 

 pieces of iron, and a battery and a coil of wire with which to produce 

 any desired magnetic field, and any number of measuring-instru- 

 ments, he would find it extremely hard to select something to measure 

 that might yield a coherent and intelligible set of data. He would be 

 able to show in a vague way that the greater the magnetic field acting 

 upon any piece of iron, the more powerful a magnet it becomes; but 

 if he were to search for some precise measurable quantity that could 

 serve as a measure of the power of the magnet, and that would be 

 characteristic of iron as a substance and not merely characteristic of 

 individual pieces of iron as individuals, his search would be a long one. 

 From what I have just called "the theory" he would find out what to 

 measure, and how to calculate from it the value of something character- 

 istic of iron and not affected by the shape of the pieces; he would find 

 out how to trace an "/-vs-//" curve. This curve would serve in turn 

 as a basis for theories of ferromagnetism ; but theory would have 

 entered already into the preparation of the curve. I shall therefore 

 devote the first section of this article to the principles according to 

 which such curves are determined from the immediate data. Any 



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