12 



ELECTRO-MAGNETISM. 



needle will revolve in perpetuity in the 

 same constant direction. It is obvious, 

 however, that in the circumstances under 

 which an experiment of this kind can 

 be made, this can never happen, because 

 the wire being a solid substance, and 

 passing perpendicularly through the 

 plane of the circle in which the needle 

 turns, its presence must arrest the mo- 

 tion of the needle as soon as it comes 

 in contact with it. The only position 

 which the needle can take, therefore, is 

 that of resting against the wire in the 

 manner represented in fg. 16. In any 



Fig. 16. 



other part of the circle, it will move on- 

 wards in the direction indicated by the 

 arrows. 



(36.) Having thus investigated the 

 action of an electric current on a single 

 pole, we are now prepared for the consi- 

 deration of its combined action upon 

 the two opposite poles of a magnetized 

 needle, balanced in the ordinary way 

 on its centre. In this case, the current, 

 descending through the wire W, fig. 1 7, 



Fig. 17. 



-.T) 



exerts a contrary action upon the two 

 poles, N and S, of the needle. When 

 the needle is in the position PQ, that is, 

 in the same line with W, these two con- 

 trary forces, acting at right angles to 

 the radius, and on opposite sides of 

 the centre, concur in their rotatory ef- 

 fect, and the needle is urged by the sum 

 of these forces to turn in the direction 



indicated by the arrows placed at 

 these points. When the needle is in the 

 position SN, at right angles to the line 

 WC, the rotatory forces, being directed 

 perpendicularly to WS and WN, as 

 indicated by the arrows, oppose one 

 another, and acting by the levers C* and 

 Cn, which are equal in length, are in 

 exact equilibrium. The equilibrium is 

 stable, as will be evident from consider- 

 ing that the displacement of S, in the 

 direction of D, increases the length of 

 the lever Cs, while the accompanying 

 motion of N towards E, diminishes the 

 length of Cn. The force represented by 

 the former, will, therefore, preponderate 

 over that represented by the latter, and 

 will carry back the pole S to its former 

 situation. The same would happen, 

 were the displacement made on the 

 other side of S, for in that case, the 

 force which impels the pole N would 

 have the advantage over that which acts 

 on the pole S, 'and would restore the 

 needle to its position of rest SN. This 

 opposition of forces occurs when the 

 needle is situated any where between the 

 lines AB and DE, which are respectively 

 perpendicular to the tangents to the 

 circle, WA and WE ; for, in either of 

 these situations, AB or DE, the rotatory 

 force exerted in one of the ."poles, is, as 

 we have before seen, 30*, reduced to 

 nothing. Beyond these positions, the 

 rotatory force changes its direction, 

 so that in any part of the arcs APE, 

 and DQB, the forces acting upon the 

 two poles conspire in producing a si- 

 milar effect of rotation. 



(37.) In proportion as the wire W, 

 fig. 18, is brought nearer to the needle, 

 the arcs ASD and ENB, in which the 

 two forces oppose each other, form a 

 larger portion of the circle, while those 

 in which they concur, AE and DB, 

 become smaller. Here it may also be 



Fig. 18. 



