ELECTRO-MAGNETISM, 

 mined by the relative direction of the round on an axis, A X, be subjected to 



current. 



Fig. 128. 



(220.) A hollow cylinder, fig. 129, 

 balanced in a similar manner as that 

 of fig. 123, may be substituted for the 

 wire; but its revolution will in general 



Fig. 129. 



not be so rapid as the wire, because, 

 although it may convey a more powerful 

 current, the weight to be moved is 

 also proportionably increased. 



(221.) The rotation of the circular or 

 spiral conductor may in like manner be 

 exhibited by suspending either of them 

 from a point, in the axis of the circle, as in 

 fig. 130, and subjecting it to the action of 



Fig. 130. 



a terminated vertical current, that does 

 not extend to both sides of its plane. 



(222.) The effect is the same at what- 

 ever angle the direction of the straight, 

 current is inclined to the plane of the 

 circular current, provided the axis on 

 which it revolves be perpendicular to 

 that plane and pass through its centre. 

 Thus, if the wire AB,/g-. 131,moveable 



the action of a circular current C D, the 

 plane of which is wholly below it, it will 

 revolve, describing a conical surface. 



Fig. 131. 



(223.) Pursuing this investigation, it 

 becomes evident that a revolving mo- 

 tion will equally take place, if the 

 straight conductor be parallel to the 

 plane of the circle, provided it does not 

 exceed in length the radius of the circle. 

 Thus, the straight conductor A B, fig. 

 132, which is wholly within the circle 

 C D, revolves round the axis A, in the 



Fig. 132. 



same direction as the current in that 

 circle, when its own current is passing 

 from the circumference to the centre. 

 When its current passes from the centre 

 to the circumference, it will revolve in a 

 direction contrary to the motion of the 

 current in the circle. On the other 

 hand, if the straight current be fixed, 

 and the circle moveable round its centre, 

 the action of the former will cause the 

 latter to revolve in directions opposite 

 to those which have been just stated. 



(224.) It may be observed that we 

 have limited this proposition to the cases 

 in which the current A B does not ex- 

 tend beyond the circumference of the 

 circle; for if it did, as seen in/g-. 133, 

 the exterior portion A B, being affected 

 in an opposite manner from the interior 

 portion B C, there would arise an op- 

 position among the rotatory forces ; 

 and the amount as well as direction of 

 the resulting motion would be regulated 

 by the difference between them. This 



