



91. "I On Variational Electric and Magnetic Screening. 421 



of frequency. This is just what is to be expected from the fact that 

 broad enough and long enough iron plate exercises a large magneto- 

 ,tic screening influence ; which, with a thick enough plate, will be 

 nearly complete that comparatively little is left for augmentation 



of the screening influence by alternations of greater and greater 



frequency. 



5. A copper shell closed around an alternating magnet produces a 



screening effect which on the principle of 3 we may reckon to be 



little short of perfection if the thickness be 2^ cm. or more, and the 



frequency of alternation 80 per second. 



16. Suppose now the alternation of the magnetic force to be pro- 

 duced by the rotation of a magnet M about any axis. First, to find 

 the effect of the rotation, imagine the magnet to be represented by 

 ideal magnetic matter. Let (after the manner of Gauss in his treat- 

 ment of the secular perturbations of the solar system) the ideal 

 magnetic matter be uniformly distributed over the circles described 

 by its different points. For brevity call I the ideal magnet sym- 

 metrical round the axis, which is thus constituted. The magnetic 

 force throughout the space around the rotating magnet will be the 

 as that due to I, compounded with an alternating force of 

 ich the component at any point in the direction of any fixed line 

 ies from zero in the two opposite directions in each period of the 

 rotation. If the copper shell is thick enough, and the angular 

 velocity of the rotation great enough, the alternating component is 

 almost annulled for external space, and only the steady force due to I 

 is allowed to act in the space outside the copper shell. 



7. Consider now, in the space outside the copper shell, a point P 

 rotating with the magnet M. It will experience a force simply equal 

 to that due to M when there is no rotation, and, when M and P 

 rotate together, P will experience a force gradually altering as the 

 speed of rotation increases, until, when the speed becomes sufficiently 

 great, it becomes sensibly the same as the force due to the sym- 

 metrical magnet I. Now superimpose upon the whole system of the 



| magnet, and the point P, and the copper shell, a rotation equal and 

 opposite to that of M and P. The statement just made with refer- 

 ence to the magnetic force at P remains unaltered, and we have now 

 a fixed magnet M and a point P at rest, with reference to it, while 

 the copper shell rotates round the axis around which we first sup- 

 posed M to rotate. 



8. A little piece of apparatus, constructed to illustrate the result 

 experimentally, is submitted to the Royal Society and shown in action. 

 In the copper shell is a cylindric drum, 1*25 cin. thick, closed nt 

 its two ends with circular discs 1 cm. thick. The magnet is sup- 

 ported on the inner end of a stiff wire passing through the centre of 



Herforated fixed shaft which passes through a hole in one end of 

 DL. XL1X. 2 F 



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