302 R. Sissingh on Kerr's 



could be displaced by the observer in a vertical direction in 

 order to bring the crater of the positive pole always opposite 

 the slit of the collimator. 



The mirror-magnet was demagnetized by alternate currents 

 whose absolute mean value was approximately zero. By 

 inserting a resistance constructed out of electric-light carbons, 

 the current could be reduced to 2*3 amperes. The ring was 

 then inserted in a shunt-circuit, by which means the strength 

 of the current through it could be reduced to 0*02 ampere. 

 This arrangement was carried out by means of a switchboard, 

 and is shown in fig. 3 with commutator, amperemeter, and 

 two plug-switches. The key of the commutator turns round 

 the point 9 and connects together either 1 and 2 and 6 and 7, 

 or else 1 and 3 and 4 and 5. The magnetizing current passes 

 from A to B, e (or /), C, plug d 2i V, E, 1, 2, ring, 3, 6, 7, 8, 

 F, plug b 2 , G. If the ring is in the shunt-curcuit the main 

 current passes along A, B, /, C, plug c 2 , the shunt H, K, L ; 

 a, M, N, 1, 2, ring, 3, 6, 7, 8, F, P, plug c } , T, R. 



11. Test of the Method of Observation. — If the amplitude /jl, 

 and not the phase m, of the magnetic component of the light 

 alters its sign with the direction of magnetization, we need 

 not determine the angle <jf>, but only ty (§§ 4, 5). The deter- 

 mination of the angle (/> requires a knowledge of the principal 

 positions of the Nicol prisms, in which the planes of polariza- 

 tion are parallel or perpendicular to the plane of incidence. 

 This would always necessitate a demagnetization of the mirror 

 on account of permanent magnetism. First of all we shall 

 show that on demagnetizing, as in § 10, the principal positions 

 of the Nicol prisms are the same, whatever sign the smallest 

 magnetizing current of 0*02 ampere may have. The per- 

 manent magnetism therefore exerts no influence on the 

 reflexion. 



emagnetization by Currents 



The Mirror demagnetized afresh 



in alternate directions of 



Alternate Currents of 



+15 amp. to +002 amp. 



+15 amp. to — 0-02 amp. 



Principal positions of the 



Principal positions of the 



Polarizer. Analyser. 



Polarizer. Analyser. 



144-7 304-0 



144-9 304-9 



143-8 304-7 



144-5 303-3 



144-0 303-9 



144-9 305-8 



145-1 304-0 



145-2 304-8 



145-0 305-3 



144-5 303-9 



145-3 305-1 



144-7 303-8 



145-0 304-5 



145-4 304-4 



145-1 303-8 



144-7 303-8 



144-75 3044 144*8 304-3 



