Magneto-optic Phenomenon. 255 



I desire to thank Messrs. Hartmann and Braun, of Bocken- 

 heim and Frankfort, and their foreman, Mr. G. Troll, who 

 executed this difficult job for me with perfect success. The 

 ovoids were magnetized in a coil of 1080 turns, 30 centim. 

 long. In order to let light pass, small looking-tubes (*7 

 centim. diam.) had been fixed between the wire during wind- 

 ing. These lay in meridian planes through the coil's axis, 

 and inclined at various angles to it. 



§ 4. Notwithstanding the resulting slight irregularity of 

 winding, the field was sufficiently uniform over the space 

 to be covered by the ovoids. This was tested by the mag- 

 netic rotation in bisulphide of carbon, measured at different 

 points on the coil's axis. At the same time the field could 

 thereby be determined in absolute measure ; and a numerical 

 factor deduced, by means of which the readings of an ampere- 

 meter in the coiFs circuit were afterwards reduced to fields. 

 The ovoids' moment was magnetometrically determined in 

 the ordinary manner, the coil's action on the magnetometer 

 being compensated by that of a second coil. Apart from ovoids, 

 I have also worked with small reflecting disks, fixed between 

 the poles of a Ruhmkorff electromagnet. Certain other 

 arrangements will be described in due time. 



§ 5. Notations, often recurring : — «£j, intensity of coil's 

 field ; 3? magnetization, and 3 , mean magnetization of 

 ovoids ; 2 a, 2 6, their major and minor axes; l\ = b/a, axial 

 ratio ; 9?, outward normal to mirror ; a, angle <fc (3, 9?). K, 

 Kerr's constant (see § 24) ; 6, temperature ; X, wave-length; 

 e , rotation on " polar" reflexion ; e, rotation (according to 

 § 2 this is to be understood as meaning the angle between the 

 incident rectilinear vibration and the major axis of the reflected 

 ellipse) . Of course, 2 e was in the first place determined ex- 

 perimentally by reversing the current, generally as a mean 

 of from 10 to 30 observations of azimuth. In every case 

 below, however, the values of e are given in minutes, radian 

 measure being still too little usual in such work. Otherwise, 

 wherever the contrary is not expressly stated, all quantities 

 and scales of diagrams are given in C.Gr.S. units. Wherever 

 the sense of rotations has to be expressed with reference to 

 vectors, I shall use the system of the vine and European 

 " right-handed screw " *. Accordingly, the sign of Kerr's 

 effect, e. g. for iron, is negative, the sense of rotation having 

 to be referred to the direction of magnetization. 



* Maxwell, Treatise, 2nd ed. i. p, 24, 



