CONTEMPORARY ADVANCES IN PHYSICS 



355 



a = M{II + nI)lkT, 



(14) 



which is transported bodily into the function L{a) of equation (11). 

 This is a very abstract way of putting the fact; but the more concrete 

 ways have not been satisfying. One may say that the true field 

 acting within the material is not //, but (// + nl) — that the actual 

 though unverifiable field acting at any point in the inaccessible 

 interior of the magnet is the sum of the field He due to objects in the 

 external world, and the field Hi due to the "demagnetizing efTect of 

 the poles," and an additional term proportional to the- intensity of 

 magnetization at the point in question. The suggestion of Weiss, then, 

 is tantamount to making a new assumption concerning this tantalizing 

 internal field. The natural next step is, to visualize or explain the 

 agent of the extra force, the "molecular field" as Weiss calls it; that 

 is the step which no one has yet succeeded in making, not at least 

 with general assent. 



Making the expression in (14) the argument of L{a), we see that 

 the fundamental equation (11) now has the variable / in both its 

 members, and must be solved for /. The resulting function is one 

 of the infinitely many which have neither names nor well-known 

 features, and most of those who write on this subject recommend 

 the high-school expedient of plotting the curves representing the 

 two functions 



/ = (kT/nM)a - H/n, (15a) 



/ = Ima.Ma) = NML{a), (156) 



in a coordinate-plane with / as ordinate and a as abscissa, and looking 

 for the point or points of intersections between the two curves. These, 

 which I shall designate for a few paragraphs as "the line" and "the 



Fig. 13 — The "curve" and the "Hne" of the Langevin-Weiss theory 

 of ferro magnet ism 



curve," are shown in Fig. 13. It is easy to see that, when T is held 

 constant and // increases, the line slides from left to right and the 

 intersection-point mounts along the curve; when // is held constant 



