PRODUCED BY ALTERNATING ELECTRIC CURRENTS. 
319 
23. Hence 
(a.) If cr, cr' or c vanish, the couple vanishes, as might have been expected. 
(/ 3 .) The signs of the couples fall into three cases :— 
(i.) When 6/i is positive and > d, both couples are positive, and tend to increase </>. 
Fig. 36. 
Fig. 37. 
m 
Iy 
(ii.) When 6h is numerically less than d, the a, b couple is positive, and the a 
negative. 
Fig. 38. 
v 
SI 
(iii.) When 6h is negative and numerically > d, both couples are negative. 
Fig. 39. 
f w 
i 
