336 CONSTITUTION OF MAGNETS. 



From which follows, taking the half sum of these expressions 



to 



or 



The relative potential energy W of the two shells is equal to the 

 work which the forces can perform when one of the shells C moves 

 to an infinite distance. We have then 



(33) 



353. This expression for the energy may be put under several 

 different forms. 



We have, in fact, 



3-' 



W/>__!**;_ _ cosecosff = _:* ' 



os os ^rosos ^r 405 os 



We may then write 



'o- 



117 ,^'f f COS(9cOS(9 '^,7> AA'ff * r r jJ' 



W = <M>' - ^y = $$ r -^-, ^f^f ' 

 JJ r JJ ^^ 



Integrating this latter expression by parts, we have 



