Eleciromagnetic Effect of Convection- Currents. 451 

 .*. total force due to the disk of radius c 



, ^^a-i'" <?dc 





= 4-77^ -f^ < ^ — X — 2x V =477-^ -^ . K\ 



and for the two disks acting in the same direction, total force 



This gives the force on the lower needle. 

 Correction for the upper needle : — 

 Potential at any point due to a circular current, 



-23- equals the solid angle subtended at the point by the circle 



(i=1.2.3...) 



Substituting the value of I, we have as the potential of the 

 disk 



But 



^' *■ '' i.2...ia^'W 



and 



