348 



Sir William Thomson 



[April 10, 



at the same potential as the grating, it takes no sensible proportion 

 of the electricity from the grating, and experiences no sensible force 

 when its distance from the grating exceeds a limit depending on the 

 ratio of the diameter of each bar to the distance from bar to bar. 

 The mathematical theory of this action was partially given by 

 Maxwell,* and yesterday I communicated an extension of it to the 

 Eoyal Society. 



II. 



Magnetostatic screening by soft iron would follow the same law 

 as electrostatic screening, if the magnetic susceptibility of the iron 

 were infinitely great. It is not great enough to even approximately 



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Fig. 6. 



fulfil this condition in any practical case. The nearest aj^proach to 

 fulfilment is presented when we have a thick iron shell completely 

 enclosing a hollow space, but the thickness must be a considerable 

 proportion of the smallest diameter, not less than -j^^^, perhaps, for 

 iron of ordinary magnetic susceptibility to produce so much of screen- 

 ing effect that the magnetic force in the interior should be anything 

 less than 5 per cent, of the force at a distance outside, when the shell 

 is placed in a uniform magnetic field. The accompanying diagram, 

 Fig. 6, representing the conning-tower of H.M.S. ' Orlando,' and 

 the position of the compass within it, has been kindly sent to me by 

 Captain Creak, R.N., for this lecture, by permission of the Controller 



* Arts. 203-205. 



