300 



MEASUREMENT OF CURRENTS. 



the opposed pile E, and the key K. If the resistances R and Rj 

 have no induction of their own, then if V is the difference of 

 potential at the two points A and B, I the intensity of the prin- 

 cipal current, I' that of the shunt current on contact, and, finally, 

 E the electromotive force of the opposing battery, we have 



and therefore 



914. OPTICAL GALVANOMETER. The action of a magnetic field 

 on polarized light (591) also furnishes a means of measuring the 



C.L.CORY, 



A 



Fig. 176. 



intensity of the current. Suppose that a ray of polarized light 

 traverses, in the direction x l9 the thickness x l -x Q e of a body 

 for which Verdet's constant (594) is <o, the rotation 6 of the 

 plane of polarization is expressed as a function of the component 

 of the field, parallel to the direction of the ray, by the formula 



(57) 



,(V-V') = 



or, if F is the mean value of this component for the length e t 



