ELECTROMAGNETIC EFFECT OF CONVECTION-CURRENTS 257 



Correction for the upper needle : 



Potential at any point due to a circular current, 



V'= Cldw, 

 equals the solid angle subtended at the point by the circle 



Substituting the value of /, we have as the potential of the disk 



'* * 



a. 4.. .81 1M 



/_v 1.3...(2i-l) p /c\"l 



( ; a.4...2Ha*+2) W J 



But 



and 



8 p _' 



& ft 



.'. The force 



f _atc". 

 \ ~^^ 



and for the two, 



where the sign of the entire expression has been changed, since the 

 poles of the upper and lower needles are opposite. 

 Or 



X_Q_ * Z? 

 i or. ^. 



17 



