298 



H. E. J. G. du Bois on Magnetization in 



magnetic curve to rise much more rapidly at first without, 

 however, attaining in the end to values beyond those of the 

 unannealed metal. The yellow-tempered hard steel was further 

 tempered dark blue and finally annealed at red heat without 

 any very marked change in the general character of the curve 

 ensuing. 



The values of ®, found for iron and nickel, are 216vl and 

 54'0 respectively at 25°, as interpolated on the temperature- 

 curves of fig 1. Thin sheets of electrolytic iron and nickel 

 were now prepared in the shape of oblong ellipses and 

 weighed. They were subjected (at ordinary temperature) to 

 a field of 900 C.G.S., and their moments approximately 

 measured with the following results : — 



a. Iron, about *05 centim. thick, originally intended for 

 printing Russian banknotes, and kindly given to me by 

 Prof. F. Kohlrausch : (5 = 211*4, 30 per cent, of which was 

 residual. 



b. Iron, about "005 centim. thick, which I electrolytically 

 obtained from the so-called Varrentrapp bath: @ = 213'7, 

 50 per cent, of which was residual. 



c. Nickel, about "015 centim. thick, from citrate of nickel : 

 (5 = 53*5, 20 per cent, of which was residual. 



Electrolytic cobalt could not be got in films sufficiently 

 thick for this purpose. The results appear to warrant the 

 conclusion that with these very strong magnetizing fields 

 much does not depend on the quality of the metal, as long as 

 it is tolerably pure. Nor has its condition, as resulting from 

 the mechanical and thermal treatment previously applied, 

 any sensible influence. Of course the experiments in this 

 paragraph are meant to be novel only with respect to the 

 strong fields applied. 



II. Magneto-optic Obsekvations. 



§ 9. Method and Apparatus. — As the isthmus method is 

 based on the principle of tangential continuity of .0 and dis- 

 continuity of $ 00' = -0 and S8'>*8), so the method to be 

 described may be said to rest on the normal discontinuity 

 of J$ and continuity of $ ($ < .0 and 2?' = 2?). In addition, 

 the normal component of 3 is measured by the rotation 

 impressed upon plane-polarized light normally reflected from 

 the magnetized metal. In order to obtain the former the 

 latter has only to be divided by a factor, which I have 

 proposed to call " Kerr's constant " (loc. cit. § 24). 



The optical apparatus used for accurately measuring the 

 rotation of red light has been described at length (loc. cit. 



