352 



ELEMENTARY LESSONS ON [CHAP. ix. 



resolved part of the magnetic force in the direction of 

 the ray ; and H cos /3 I is the difference of magnetic 

 potential 1 between the point A where the ray enters and 

 B where it leaves the medium. Hence /, the coefficient 

 of specific magnetic rotatory power, is found by divid- 

 ing the observed angular rotation by the difference of 

 magnetic potential between the points where the ray 

 enters and leaves the medium ; or 



- V B - V A 



Different substances possess different magnetic rotatory 

 powers. For diamagnetic substances the coefficient is 

 usually positive ; but in the case of many magnetic 

 substances, such as solutions of ferric chloride, has a 

 negative value ; (i.e. in these substances the rotation is 

 in the opposite direction to that in which the magnetising 

 current flows). The phenomenon discovered by Hall 

 (Art. 337) appears to be intimately related to the 

 phenomenon of magneto-optic rotation. 



It is convenient, for purposes of reference, to take the 

 rotatory power of bisulphide of carbon as unity. Careful 

 measurements executed by J. E. H. Gordon have shown 

 that the rotatory power of bisulphide of carbon, thus 

 assumed as a standard, must be multiplied by 3*047 x 

 io~ 5 to reduce it to C. G. S. measure; for he finds that 



1 For force X length = ivork; and the work done in bringing a unit 

 magnetic pole from A to B against the magnetic force measures the 

 difference of magnetic potential. See Art. 310 (e) 



