256 H. du Bois.tm Kerr's \ Magneto-optic Phenomenon. 



First Paet. 



§ 6. I begin by describing the experiments with the ovoids. 

 The polarizer consisted of a nicol and a Hartnack-Prazmowski's 

 semi-prism (see § 16). The zirconia disk stood in the focus of 

 the first lens, which thus projected parallel rays on to the 

 mirror. Distance of vertical scale about 175 centim. 



§ 7. Normal temporary magnetization. — The 5 ovoids used 

 in this case were provided at one end with circular mirrors 

 perpendicular to the axis of revolution, and were magnetized 

 parallel to the latter. In this case, therefore, the reflexion, 

 according to Dr. Kerr's nomenclature, is " polar," i. e. the 

 metal is magnetized normally to the mirror (a = 0). An 

 ellipsoid (particular cases : sphere, spheroid, ovoid) is known 

 to be the only finite figure in which iron is uniformly mag- 

 netized when subjected to a uniform field. In particular, no 

 figure bounded by planes exists, for which this might be the 

 case otherwise than approximately. Instead of such of vanish- 

 ing size I had to use mirrors whose dimensions were only so 

 far reduced as was consistent with sufficient intensity of the 

 reflected beam. The ovoidal shape was necessarily disfigured 

 by the missing segments ; and the magnetization, uniform on 

 the whole, must be differently distributed in the neighbour- 

 hood of the small planes ; it is easy to see that it must be less 

 than with an infinitely small mirror. 



§ 8. Some preliminary trials were made to show that this 

 is the correct view of the case. On turning down the mirror 

 of one ovoid successively from *7 to '5 and then to '3 centim. 

 diam., the rotation increased each time, cceteris paribus. 

 Another ovoid, provided with mirrors at both ends, showed 

 the lesser rotation at the larger mirror. Lastly, an ovoid 

 was fitted as one pole-piece into an electromagnet, with the 

 result that the rotation, for about the same estimated mean 

 degree of magnetic saturation, proved considerably larger than 

 when the ovoid was magnetized in the coil. The second 

 bored pole-piece acts in the same manner as Kerr's " sub- 

 magnet," i. e. it prevents the self-demagnetizing action of the 

 plane end. In the final experiments the observed rotation 

 may be estimated at 60 to 75 per cent, of that which would 

 occur were the mirror of vanishing size (by data of § 19). 



§ 9. The rotation, evidently a local action of magnetiza- 

 tion, was now directly compared with the magnetometer- 

 deflexion, apparently an action at a distance. The quotient 

 (moment/volume) gives the mean magnetization 3' °f * ne 

 ovoid, plotted in the broken curves of fig. 1 as a function of 





