CONTEMPORARY ADVANCES IN PHYSICS 309 



field-measuring devices and the observations made by the method of 

 the loop are all reconcilable with one another, provided that the 

 vector B is made parallel to / and H and equal to 



B = H + 47r/. 



The vector B is known as the induction. The relation between B 

 and H is often plotted instead of the relation between / and H; 

 naturally if either relation is known the other can readily be found. 

 The ratio of the magnitudes of B and H is called permeability and 

 denoted by m; the ratio of the magnitudes of / and H is called suscepti- 

 bility and denoted by k or a or %• 



One might think that this quantity B should be identified with the 

 magnetic field which is supposed to exist within the metal and to 

 magnetize it. Though all the textbooks beseech the student not to 

 confuse the induction with the field (he is usually asked to imagine 

 himself digging variously shaped infinitely small holes within a magnet, 

 and putting an instrument into each to measure the magnetic force 

 inside it), the distinction has an obstinate way of not becoming clear. 

 We should get just as self-consistent sets of curves if we were to plot / 

 against (He -f i^j -f 47r/) as we do when plotting / against (He + Hi) ; 

 it would merely be tantamount to adding iir to the "demagnetizing 

 factor." As a matter of fact nearly everyone, as soon as he begins 

 to theorize about the state of affairs inside magnetized bodies (or 

 polarized dielectrics), promptly assumes that the acting field is some- 

 thing different from the resultant of He and Hi. Some make it equal 

 to (// -f ^ttJ), attributing the term f ttZ to an action of the molecules 

 which are neither very close to nor very far from the point where the 

 field is being evaluated. Some (Weiss and his many followers) make 

 it equal in ferromagnetic metals to the sum of H and a term nl, the 

 factor n being so enormous that the postulated field is millions of times 

 as great as // and thousands of times as great as B. The extra field, 

 they say, is "not magnetic"; but this distinction is more obscure 

 than the other. Nobody really knows what the field inside a magne- 

 tized solid is. The best policy is to continue plotting / and B as 

 functions of H, regarding H as the independent variable sanctioned 

 by tradition. 



B. The Relation between Intensity of Magnetization 



AND Magnetic Field 



Since all of the actions of magnets are interpreted by supposing 



that in every magnetizable substance the intensity of magnetization 



is controlled by the magnetic field in a definite and peculiar way — 



