72 POTENTIAL ENERGY OF A DISPLACEMENT. 



the energy expended in giving the body first a twist 

 of amplitude 6' about 9, and then a twist of amplitude ^' 

 about 0, is to be represented by 

 Fofff* + Fv^. 



By taking a third screw, conjugate to both 9 and <p, 

 and then a fourth screw conjugate to the remaining three, 

 and so on, we see finally that the potential reduces to 

 the sum of n square terms, when each pair of the screws 

 of reference are conjugate screws of the potential. 



