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BELL SYSTEM TECHNICAL JOURNAL 



and T increases, the line wheels counterclockwise around the point 

 where it cuts the axis of abscissae, and its intersection with the curve 

 descends along the latter. 



There is a valuable approximation, which is more nearly valid, the 

 higher the temperature and the lower the field. At the origin, the 

 tangent to the curve ascends with slope NM/S (as I have said) and 

 so long as a is not greater than unity, the ordinate of the curve agrees 

 within six per cent with the ordinate of the tangent. If H is so 

 small and T so great that the crossing of the line and the curve occurs 

 within this range, the problem of locating it may be translated for 

 all practical purposes into the algebraic problem of solving the simul- 

 taneous equations 



/ = {kTlnM)a - H/n, I = NMa/S, 



achieving which, one obtains 

 //// = : 



C 



T - Q' 



C = NAP/Sk 

 e = nC. 



(16) 



(17) 



The susceptibility of an assemblage of elementary magnets, in thermal 

 equilibrium under the influence of an applied field on which there is 

 superposed an extra field proportional to the magnetization of the 

 assemblage, should then depend on temperature approximately 



Fig. 14 — Susceptibility-vs.-temperature curves for iron, cobalt, and nickel above 

 their respective Curie-points. (After P. Debye.) 



