( 733 ) 



QüI'T, denoting by S^' a certain mean value of the A'-component of 

 tlie E. M. F. within r. We may now write 



Similai'ly 



an integral that has to be used for negative values of z' less 

 than — Aj. 

 § 6. If lastly 



the point F lies between the tangent planes and both integrals difter 

 from zero. We find by some transformations 



s' i-^z' 27t:' 'i-nz' z' 2Tr:' 



j-J7^' ''=^T' -*TF^^'^ -^^^^- <i.' 



— Ai — /ii 



z' 2nz' 2tj' 2n:2' 



Jt?^^ ^^'=^« +«y7®^- - 



4jr' r 



2nj' 



So that in this case the ^'-component of the electric force is 

 given by 



i-xz 



„ id(o r 4jr 4:7r' ~' TV C^^ ' TV , , 



8;rTF[_ TV T'V' J 



inz' hi ,2m' 



_^^^Tvr -tt^^A 



T'V' J J 



z' 



Since z' lies between — h^ and A,, we may replace the exponential 

 factors by 1, both before and behind the sign of integration. Then 

 we find finally 



Is'. If P lies between the tangent planes 



