PRODUCED BY ALTERNATING ELECTRIC CURRENTS. 
325 
The couple on the a, b' shell is 
36 TTp^o^a/^c a a' 
" p 3 p' 3 A 1 A\ d? 
(26 + b'). 
27. Thus we see that 
(a.) The couples vanish (as they should) when c = 0. 
(/3.) When 6 = — b' and a — a, the figure is symmetrical to plane YOZ, and the 
couples will be equal and opposite (as they should): that on the a, b shell being 
positive if cl be positive. 
Fig. 47. 
(y.) For other cases the discussion of sign is similar to that for the current I cos pt, 
and there are three cases :— 
I. When 6 and b' are positive, the couples are both negative, as also if 6' be 
negative and t (6 + 6') > d/6. 
Fig. 48. Fig. 49. 
i 
II. When ^ (6 + b') < d/6 numerically, the signs are + 5 —. 
Fig. 50. 
