326 MAGNETISM 



Rotation by films of iron, nickel, and cobalt. In 



1884 Kundt * succeeded in depositing on glass films of iron, nickel, 

 and cobalt Jess than a wave length in thickness which easily 

 transmitted light. When one of these was placed in a magnetic 

 field normal to the lines of force the plane of a polarised ray 

 passing through was rotated in the positive direction i.e. the 

 direction of the magnetising current and by an amount pro- 

 portional to the thickness of the film. The rotation increased with 

 increase of the field up to about 17,000 ; but when the field exceeded 

 this the rotation did not increase much. The maximum rotation 

 of a certain red ray in iron was such that passage through 1 cm. 

 of iron would at the same rate give a rotation of 200,000. 

 The rotation of blue light was less than that of red. 



The approach of the rotation to a limit suggested that the 

 rotation is proportional, not to the external magnetising force H, 

 but to the intensity of magnetisation I. With H = 17,000 we may 

 suppose the iron to be saturated or that I has reached its limit. 

 In an ordinary non-ferromagnetic substance the intensity of 

 magnetisation is given by I = /cII. wheiv K is a minute constant 

 and I is proportional to H. In a ferromagnetic metal K is not 

 constant and I is not proportional to H. Du Boisf plotted I 

 against H for each of the three metals iron, nickel, and cobalt, 

 using Rowlands'^ values. He then plotted rotation for a definite 

 thickness against H, using the rotations determined by Kundt and by 

 himself, and found that the form of the two sets of curves was the 

 same, or the rotation was proportional to I. 



Representation of the rotation of the plane of polari- 

 sation by two equal circularly polarised rays -with 

 opposite rotations travelling with different velocities. 

 Let a point 1* move with uniform angular velocity w in a circle 

 radius a. Drop a perpendiculai PN on a iliameh r ACA' It' 1* 



starts at t = from A, ACT = o> and 

 CN = a cos tot. Thru N has a simple 

 harmonic motion with the same period 

 as P. 



When P starts from A let P' start 

 from the same point with the same 

 angular velocity 01 round the same circle 

 but in the opposite direction, so that P' 

 is always in PN produced. Then the sum 

 FlQ 241> of the displacements of P and P' parallel 



to AA' = %a cos wt, and the sum of the 



displacements perpendicular to AA' is /ero, since the displacements 

 are equal and opposite. Hence a simple harmonic vibration may 

 be made up of, or conversely may be resolved into, two circular 

 motions, each of half its amplitude and with equal period. 



* Phil. Mag. [5], xviii. (1884), p. 308. 

 t Ibid. [5], xxiv. (1887), p. 445. 



