324 MAGNETISM 



shadow"" method,* on the general principle now alwav- adopted in 

 polarimctry, in which the field is divided into n gion- with their 

 planes of polarisation slightly inclimd to <a<h nthcr. 

 sodium light used was polarised bv a Nicol. It tin h 

 through a cell containing sugar solution, but a plate of g' 

 immersed in the solution, and placed -,> that one half of tin- 1> am 

 passed through it, while the other half passed bv its , 1 so 



through an extra thickness ot-ugar. solution. This half of the h- 

 was therefore a little more rotated than the half passing thru 

 the glass, and when the beam emerged from the cell its two h > 

 were polarised in plain-- making a small angle with each n* 

 Then the beam pa--, d through the tube and in' -ing 



Nieol in front of the obsi I i cither fonissed 



his eye directly on to the the glass plate in the -ugar cell, 



or viewed tl, G -pc. To un tin- 



method let us stippo-e that, to begin with, there : t in 



the coil. Let OK OL, Fig. .M, r pic-nit the c<|iial amplitudes and 



N O M 

 239. 



the directions of vibration in the two hai he field , 



The angle K()L is .neatly exag :i tl,e li. : :;,. I 



more than <> or I* degrees OSUIuTy, 1 .. t the analvsing NIC.., 

 turned so that it extinguish :i ,,s in the r 



the bisector of KOL. If M< ).\ il ,-, pendicular to (}('. and 

 KM LN are perpendicular to it, the . when placed M 



it would extinguish vibration- < < d vibrations 



OM ON from the two halve- ot the field. If it be now turn.-d 

 through a small angle so that the vibrations admit) \1 ON - 



Fig. 240, it is obvious that there will be a great difference in the 

 illumination of the two halves. Hence the position of the analj 

 for equality of illumination can be fixed with 



accuracy. When this has been done let the current be put on 

 so that the magnetic field is established. The planes nf OK 



* Foynting, Phil. Mag. [5], x. 



