Examples 413 



17. A unit magnetic pole is placed on the axis of z at a distance / from the centre of 

 a sphere of soft iron of radius a. Shew that the potential of the induced magnetism at 

 any external point is 



-71 



where z t w are the cylindrical coordinates of the point. Find also the potential at an 

 internal point. 



18. A magnetic pole of strength m is placed in front of an iron plate of permeability 

 H and thickness c. If this pole be the origin of rectangular coordinates #, y t and if x be 

 perpendicular and y parallel to the plate, shew that the potential behind the plate is 

 given by 



e- xt Jo(yt)dt 





where 



p = 



