INDUCED MAGNETISATION 



237 



tinuity of induction gives B x . QR = B 2 . PS, where B x is the 

 induction in air, B 2 that in iron ; 



or 



^ . PQ cos 1 = B 2 . PQ cos 2 

 whence B x cos I = B 2 cos 2 . 



Thus while the components of intensity parallel to the surface are 



M 



equal, the components of induction perpendicular to the surface 

 are equal. 



If instead of air and iron we have any two media with 

 permeabilities ^ and yit 2 , intensities Hj^ H 2 , and inductions B x B 2 , 

 we still have 



H! sin 0j_ = H 2 sin 6% 



Bj cos 1 = B 2 cos 6 2 . 



Putting Bj = /XjH^ and B 2 = /x 2 H 2 , the equation of continuity 

 of induction becomes 



j cos 1 = // 2 H 2 cos 2 



and dividing the two sides of the equation of continuity of potential 

 by these we get 



tan 



tan 0. 



2. 



We may term this the law of refraction of magnetic tubes. It 

 applies of course only to cay* in which the conditions of continuity 

 hold. 



